Some Mistakes Due to What is Per Accidens



In memory of Duane Berquist

What Is Per Aliud, or Per Accidens, and What Is Per Se

IT is an axiom of Thomistic philosophy that what is per accidens must be “reduced” or “resolved” to what is per se. In other terms, the per accidens necessarily implies the per se.1 For example, Aquinas states that “Omne quod est per accidens reducitur ad id quod est per se.”2 However, St. Thomas also states this axiom in terms of the per se and the per aliud, such that it would have this sense: omne quod est per aliud reducitur ad id quod est per se—all things which are through another are resolved to something which is through itself.3 There are four senses of per se (see Aristotle, Posterior Analytics, I.4), but the senses that concern this essay are what is per se—or through itself—because of its essence, or because of its substance, or because it is the per se cause of some effect. Since the per se and the per aliud (or per accidens) are also opposed to each other, there would seem to be as many senses of the per accidens as there are of the per se.

First, we should examine some examples of what this connection means. For instance, it is not necessary that someone who speaks Spanish also be a medical doctor, since the two characteristics as such are per accidens to each other. If they were not per accidens to each other, then one could not be a physician without also speaking Spanish, or vice versa. Nonetheless, when this per accidens unity is realized in one person, this unity must be resolved to the per se habits which make him a Spanish-speaker and a physician. Note that here, what is reduced to the per se is a per accidens unity. The two habits could also be said to be united or exist as one only per aliud, namely, through the “other” which is their subject, the substance which exists per se as a unity (the human person who is both doctor and Spanish-speaker).

Another example of the per aliud being resolved or “reduced” (led back to) the per se is found in the meaning of written words. A word in Latin or English written on a piece of paper does not have meaning of itself, i.e., in virtue of being a pattern of material marks on a piece of paper. Rather, the meaning of the word is derived from the meaning which it has in speech, which meaning is derived from the mind understanding reality. This meaning found in reality is itself derivative, but for our purposes it is such realities or essences to which the per se meaning of words such as names are led back.

Closely related to this idea is the one that nihil quod est per accidens est necessarium—nothing that is per accidens is necessary.4 This implies that all contingent things are in some way per accidens, or per aliud, and must be resolved, led back to, or caused by what is per se. However, this also means that it is necessary that the per accidens be so grounded by the per se. Nonetheless, this does not confer necessity upon what is per accidens, even though it might be mistaken as such. That is, even though the per accidens is not what is essential to that which is per se, it is necessary that the per se always accompany the per accidens. This necessary connection can cause philosophical errors.

It can be the case that the attendant, per accidens unities in reality are confused with that which is essential, or what has reality per se. This mistake is frequently taken advantage of by advertisers, for instance. It is not necessary that someone speaking with scientific authority wear a lab coat, nor is it necessary that if someone is wearing a lab coat that he can also speak with scientific authority. But this per accidens reality—a lab-coat-wearing, scientific authority—is nonetheless exploited to immediately project an ethos when one appears in a commercial for a new pharmaceutical.

There are other instances where what is per accidens leads to mistakes. This is especially so when a per accidens unity is necessarily attendant upon some act, operation, or essence, but is not part of the act, operation, or essence per se. We now turn to four examples.

Examples of the Mistake Based Upon What Is Per Accidens

1. Plato’s Forms

This fallacy of conflating what is per accidens with what is per se causes even the wise to go astray, says St. Thomas.5 One example of this is when Plato failed to distinguish the accidental and the essential and was led to positing separated substances, the Forms, as the basis for science and definitions. In this instance, there are several instances of the per accidens. It is per accidens to a form that be known by a human intellect in a universal and changeless fashion. It is also per accidens to a form that it change—“form changes” only per aliud, through the composite being that is subject to change. However, the implied Platonic argument for the separated substances would claim that it is essential that those things we know as universal and changeless also exist in that way. However, a form only has those characteristics per aliud, through being known by the human mind, even though the form is, as part of a thing’s nature, not itself subject to change. What has universality and immateriality per se, on the Aristotelian analysis, is the intellect’s concept of a thing’s nature.

2. Sensation and the existence of material things

For instance, when one actually knows something through the senses, it is necessary that one receives the object as object in one’s senses. It is per se to the act of sensation that the object is “present to” the sense power. However, it is per accidens to the object sensed, as an existing thing, that it be sensed. The lonely tree can fall in the forest and still make a sound; it still has a color as it falls, unseen. Therefore, despite its being necessary that a per accidens unity between thing, thing-as-object, and sense power arise in the act of sensation, it is no part of the essence of the thing sensed that it be sensed. Otherwise, it could not exist as not actually sensed.

It is not even essential to the thing as an object of sensation that it be actually sensed, only that it is potentially sensible. Even in such a case, a thing is potentially sensible only per aliud, or, in virtue of the configuration of the sense powers of the organism in question. Thus, colors—in themselves, or per se—are not potentially sensible to the colorblind, because of a lack or privation in the relevant sense power; likewise, a sound of a frequency of 140 kHz is not a potential object of sensation to the human ear, but it is a potential sensible for a bottlenose dolphin. In neither case does the failure of the object to be potentially sensible lead to the conclusion that the thing sensed does not exist. All we could say is that it does not exist precisely qua object relative to a certain kind of sense power.

Berkley’s idealism falls into the error of the per accidens here. This is clear from the inference he attempts to make between §3 and §7 and §9 of his Principles.6 After listing the variety of ideas that are present to our mind and which alone constitute our knowledge of things (§§1–2), he claims:

That neither our thoughts, nor passions, nor ideas formed by the imagination, exist WITHOUT the mind, is what EVERYBODY WILL ALLOW. And it seems no less evident that the various sensations or ideas imprinted on the sense, however blended or combined together (that is, whatever objects they compose), cannot exist otherwise than IN a mind perceiving them.

This argument could be called a “fallacy by pun.” The ideas do not exist “without” the mind (i.e., outside the mind), and thus they cannot exist “without” the mind (i.e., apart from dependence on the mind).

Nonetheless, Berkley then claims that two things follow: “From what has been said it follows there is NOT ANY OTHER SUBSTANCE THAN SPIRIT, or that which perceives,” (§7) and that “Hence, it is plain that the very notion of what is called MATTER or CORPOREAL SUBSTANCE, involves a contradiction in it,” (§9). Or, in his famous claim about the objects of our knowledge, “Their ESSE is PERCIPI, nor is it possible they should have any existence out of the minds or thinking things which perceive them.” (§3)

That is, the existence of the objects of our ideas (e.g., sense knowledge) are always necessarily attended by the circumstance of our actual perception, and we cannot but describe them in terms which implicate this necessary connection (as Berkley points out in §3). Yet the idealist fails to distinguish that it is per accidens to a material thing that it be sensed, and instead makes this a per se condition of that material substance. It logically follows, upon Berkley’s conflation, that the perceived object does not exist apart from the actual act of “mental perception” in Berkley’s sense. This critique is devastating to the early modern empiricists, but only to them.

3. The hierarchy of choiceworthy human goods

Another example is the argument which denies the hierarchy of moral goods, which is proposed by the new natural law theory. John Finnis claims that “each [basic good], when we focus on it, can reasonably be regarded as the most important. Hence, there is no objective hierarchy amongst them.”7 Note two things. First, Finnis takes as the necessary condition of his claim that “we focus on it,” namely, focus on the basic good in our practical consideration of what is choiceworthy. Second, it is because of this condition that there is no objective hierarchy among the basic goods (practical reflection, life, knowledge, play, aesthetic experience, sociability, practical reasonableness, and religion). He goes on to make this point at greater length:

If one focuses on the value of speculative truth, it can reasonably be regarded as more important than anything; knowledge can be regarded as the most important thing to acquire; life can be regarded as merely a precondition, of lesser or no intrinsic value; play can be regarded as frivolous; one’s concern about ‘religious’ questions can seem just an aspect of the struggle against error, superstition, and ignorance; friendship can seem worth forgoing, or be found exclusively in sharing and enhancing knowledge; and so on. But one can shift one’s focus. If one is drowning, or, again, if one is thinking about one’s child who died soon after birth, one is inclined to shift one’s focus to the value of life simply as such. The life will not be regarded as a mere precondition of anything else; rather, play and knowledge and religion will seem secondary, even rather optional extras. But one can shift one’s focus, in this way, one-by-one right round the circle of basic values that constitute the horizon of our opportunities.8

This argument from one’s “focus” on the basic goods to a denial of an objective hierarchy among them falls prey to a fallacy of the accident in the sense we have been discussing.

To draw this out, I will state the argument in an equivalent fashion, as a contrary-to-fact hypothetical. If one human good were better than another (that is, if what is good for human beings comes in a hierarchy of goods), then the better good is inherently more choiceworthy. Thus, the more choiceworthy good ought to be chosen over the less. For example, if intellectual contemplation is the highest human good, it should be chosen over other goods. However (and here we can use Finnis’ example), it is clear that there are cases where we are obligated to choose to save a life rather that contemplate the truth. Thus, if we “focus” on a good, it undermines the claim that there is an objective hierarchy among the human goods.

However, this is to argue by fallacy of the accident. It is, of course, necessary that every actual choice occur in particular circumstances. That is, when we make actual choices we must “focus” our attention upon one good rather than another and see that, in the given circumstances, that good is more choiceworthy. Someone failing to choose to save her friend’s life—provided she is able, etc.—and instead choosing to contemplate a certain mathematical theorem whilst ignoring her friend’s plight would be a grave moral failure. More generally, it is per se to a good as actually chosen that it be an object of our will in concrete circumstances. However, the circumstances are clearly not per se to that good, otherwise that good could not exist in other circumstances. Finnis’ argument, through its use of the phrase “focus on it” in relation to various human goods, occludes the actual fact that his argument hinges upon the concrete context, or circumstances, of a choice. These require that we focus on, weigh, and decide upon the goods presented to us by those circumstances.

Thus, while it is necessary to an actually chosen good that it be in particular or concrete circumstances (this situation here and now), that is only per accidens to what it means to be a human good. If the circumstances were per se, then that good could not exist in other circumstances and still be that kind or type of good. However, it is clear that the act of mathematical contemplation is of the same essential type in the hypothetical circumstance where I am choosing between contemplation and saving my friend. It is merely the case that the same essential type of good is, secundum quid, not as good due to the circumstances of my particular choice.

Consequently, it does not follow that there is no objective hierarchy among human goods merely because those goods can only be chosen in circumstances which may “reorder” their hierarchy (for the reason that the circumstances are not essential to the nature of those goods), even though circumstances are necessarily attendant to those goods as actually chosen.

4. The existence of hierarchy among living things

Another example of this fallacy of conflating the per accidens and its necessarily attendant per se roots or conditions can be found in the case of certain arguments denying the existence of a hierarchy of nobility among biological organisms. This argument is usually based upon the idea of an ecological niche. In basic terms, the niche of a biological species is the environment in which it can flourish, or rather, in which it has come to flourish given evolutionary pressures of survival and its own ancestral line’s response to those pressures.

Although the meaning of the word “niche” in ecology has substantially changed over a century of existence, its multiple meanings all revolve around the Darwinian view of ecosystems that are structured by the struggle for survival. Originally, the word meant a place in the ecosystem, in the sense of the relationship to resources, predators and habitat.9

The complex history, variations, and viability of the variations of this concept within evolutionary biology do not concern us here. The general notion of a “fit environment” is sufficient. For a given organism (due to a history of evolutionary development), certain environments are better for its survival than others. Thus, certain environments are better for the human being (Homo sapiens), or for the polar bear (Ursus maritimus), or for a common fangtooth (Anoplogaster cornuta). A niche can even be used, at a biological level of analysis, to distinguish one species from another:

Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids; rather than indicating that they have only recently diverged, the new evidence suggests more frequent mating has continued over a longer period of time, and thus the two bears remain genetically similar. However, because neither species can survive long in the other’s ecological niche, and because they have different morphology, metabolism, social and feeding behaviours, and other phenotypic characteristics, the two bears are generally classified as separate species.10

Utilizing this concept of a niche, one could propose an argument such as the following. If you were playing some sort of biological Monty Hall gameshow, and had to choose between being a human being, a polar bear, or a common fangtooth, which would you choose? The conditions of the game are that you are only concerned with survival. Of course, you would immediately want to know—or else you would consider the game unfair—what sort of environment you would be set loose in. Would you be a human being in the polar tundra? Would you be a common fangtooth flopping about on the dry dock of an East Coast city? Or would you choose a polar bear only to find yourself at crushing deep sea depths? No. Unless the veil of ignorance were lifted and the environment revealed in which “you” would be placed, the choice would be a sadistic one.

This is all well and good if the stakes are mere survival, which implies surviving as an organism in a specific environment. That is, if one is an actually existing organism, then one is necessarily living in an environment. As the evolutionary idea of an ecological niche leads us to consider, it is even the case that the evolutionary history of one’s species was shaped by the availability of that niche which led to better survival of one’s forebears. So, in the genetic history of one’s kind, and ecological niche is necessary for the coming to be of a species.

Of course, someone might try to extend the argument beyond the biological and make a broader claim, as follows. Because an ecological niche is so important, this also allows us to conclude that we cannot say that one species is objectively better than another. There would be a very straightforward analogy between such an argument and Finnis’ argument against the objective hierarchy of goods. That is, if we focus on a species qua actually living in an environment and surviving, it becomes obvious that some species are better than others; yet in another niche, this hierarchy has a different order. Yet this reordering only occurs on the condition of the given circumstances or the given ecological niche.

Thus, the fallacy in such an argument against a hierarchy of living beings would be similar situated as was the argument against a hierarchy of human goods. All things considered, there is still a hierarchy of living beings in the sense that some kinds are nobler or better than others. It would be better to be Socrates, suddenly transported into the deep sea and dying, than to be an Anoplogaster cornuta and still alive.

The fallacy, once again, is that even though it is necessary to an actually living thing of a certain species that it be alive in an environment and even have had its species evolutionarily co-determined by such an environment. It is necessary that every species have such a niche, and in such a niche that species is better than others not suited for that niche. Yet it does not follow from this that its kind does not have a limited nobility or a lower goodness all things considered. This broader scope shows that the fallacy of the accident in this argument is joined with a fallacy of conflating the simpliciter with the secundum quid. That is, in a qualified way (if one is only concerned with survival), it would be better to be a polar bear in the tundra than a human being in the tundra. However, simply speaking, being a human being is nobler than being a polar bear—even in the tundra. Likewise, this fallacy of the simpliciter and the secundum quid is also present in Finnis’ argument.


We should review our examples of philosophical mistakes which occur by conflating what is per accidens or per aliud with what is per se.

⁃          1. It is per accidens to a form that it be understood by the human mind, but necessary when this happens that the form exist (in thought) in a universal and immaterial way. So, it is a mistake to infer that this mode of existence is per se to the form as such.

⁃          2. It is per accidens to an object of sensation that it be sensed by a sense power, but it is necessary when this happens that it exist (in sense) as an object of sensation. So, it is a mistake to conclude that this mode of existence is what the sensible object is per se.

⁃          3. It is per accidens to a choiceworthy, nobler good that it be in particular circumstances which make it less choiceworthy, but necessary that a choice is made in concrete circumstances. So, it is a mistake to think that human goods are not part of an objective hierarchy.

⁃          4. It is per accidens to a nobler species that it happen to be in a niche where its likelihood of mere survival makes it worse off than a more ignoble species, even though it is necessary for any species to have a niche and have evolved in a give-and-take with an ecosystem. So, it would be a mistake to think that there is no order of nobility among the forms of organic life.

It should also be briefly noted that other serious examples of this mistake exist. One might be tempted to deny the substantial identity of a living being because of the replacement of its material parts (it is necessary that such replacement take place, but it is per accidens to that substantial being that this matter is so incorporated). One might think that, since every mover we experience is also moved, that a wholly immobile mover is an impossibility (yet it is per accidens to a mover that it be moved). It also seems likely that similar mistakes are made if one thinks that creation is necessary (i.e., that God cannot but create), or in Spinoza’s monism.

The four mistakes discussed can all be reduced to a principle as to what is per se. They are all due to a mistake in thinking through how an act is received by a potency which limits it:

⁃          1. The form when known is received in the mind.

⁃          2. The sensible thing when sensed is received in the sense organ.

⁃          3. The good when chosen is an object of our will.

⁃          4. The biological species when it exists is individuated by matter and exists in a particular environment.

Thus, one could be overly attentive to the limitation of some act by a potency—this seems to happen because the reality that is the result of such limitation is more concrete and closer to us. Received act is closer to our experience and more knowable to us. However, act as such is not potency, even though it is necessary that they be joined in the many aspects, circumstances, and things of our experience. Strictly speaking, it is per accidens to act that it be limited or received; act is, simply speaking, prior to potency. Otherwise, an unreceived act could not exist.


 1 See David Bonnette, Aquinas’ Proofs for God’s Existence: St. Thomas Aquinas on: “The Per Accidens Necessarily Implies the Per Se,” (The Hague: Martinus Nijhoff, 1972).

2 In Phys., lib. 1, lect. 14, n. 7.

3 Consider ST Ia-IIae, q. 35, a. 7, c.: “Semper autem quod est per se prius est eo quod est per aliud.” In Po. An., lib. 1, lect. 7, n. 8: “Quia semper quod est per se est causa eius quod est per aliud.” In Po. An., lib. 1, lect. 37, n. 5: “Semper id quod est per se prius est eo quod est per aliud ut causa eius, ut habetur in VIII Physic.”

4 St. Thomas, In Phys., lib. 8, lect. 9, n. 7.

5 Super Boetium de Trinitate (SBdT), q. 5, a. 2, c.: “Sed hic defectus accidit ex eo quod non distinxit quod est per se ab eo quod est secundum accidens, nam secundum accidens falluntur plerumque etiam sapientes.”

6 George Berkley, A Treatise Concerning the Principles of Human Knowledge, Gutenberg E-Text; url: <>; accessed 17 February 2019.

7 John Finnis, Natural Law and Natural Rights, 2nd ed. (Oxford: Oxford University Press, 2011) 92; my emphases.

8 Ibid., 92–93; my emphases.

9 Arnaud Pocheville, “The Ecological Niche: History and Recent Controversies,” 575, in Thomas Heams, Philippe Huneman, Guillaume Lecointre, and Marc Silberstein, (eds.) Handbook of Evolutionary Thinking in the Sciences (Dordrecht: Springer Science + Business Media, 2014).

10 “Polar bear,” Wikipedia; url: <>; accessed 17 February 2019; my emphases. The source cited by Wikipedia is Ian Stirling, Polar Bears (Ann Arbor, MI: University of Michigan Press, 1998); see pp. 22 and 26.