How is it that we can experience incoming data from the five senses as coherent, rather than incoherent, perceptions of objects and people? This issue, often referred to as “the binding problem” in neuroscience, has been a concern of philosophers since at least Plato where, in his dialogue Theaetetus (184c-187a), we encounter an argument that introduces the soul as a means to account for our coherent perceptions. This argument has become known as the Wooden Horse Argument and is one of Plato’s more impressive investigations into the nature of the psyche. In the dialogue the argument is used to show that knowledge is not only a matter of sense perception. But the argument has other far reaching consequences.
Socrates asks a character Theaetetus whether we sense with or through our senses. Theaetetus’ answer is that we sense through the senses and with our soul. Socrates agrees and asserts that if we sense with the senses this would imply “there are a number of senses sitting inside us as if we were Wooden Horses” (184d). K. M. Nielsen explains this Homeric reference to Odysseus and his men silently hiding in the Trojan Horse:
“The five men huddled inside the Wooden Horse all have separate consciousnesses, and there cannot be any unity to their experience. The men inside the horse cannot speak, since that would alert the Trojans to their plan, and they are presumably too far away from each other to use sign language. The horse itself is insentient—i.e. deaf and blind and without the ability to taste, smell, or touch. The problem with man qua Wooden Horse is, as Socrates points out, that it lacks ‘some single form, soul or whatever one ought to call it, to which all these converge—something with which, through the senses, as if they were instruments, we perceive all that is perceptible’ (184d).”
If the senses cannot be unified in some central place then there will be no unified experience or subject of experience. Moreover, it is clear we compare, contrast, judge, collect, divide, and rationally assess the data of the senses. But to do this, we must employ thoughts that employ concepts such as: existence, difference, sameness, one, two, likeness, and unlikeness. So, for example, to say that some sweet thing x is not white is to employ the concepts (1) existence (because we are asserting that x and the color white exist insofar as we are discussing them); (2) difference (because we are asserting x is not white); (3) unlikeness (because x is unlike something sweet); (4) one (because to identify x as such we must judge it to be one); (5) two (because we are dealing with two things); and (6) sameness (because x is judged to what it is and not something else; it is the same as itself). These and many other concepts will be employed in making judgments about sense data and are not drawn from the senses themselves. Francis Cornford explains:
“In these judgments the thinking mind uses terms like ‘exists’, ‘is the same as’, ‘is different from’, which are not objects of perception reaching the mind through the channel of any special sense, but are ‘common’ to all the objects of sense. The mind gains its acquaintance with the meaning of such terms through its own instrumentality, not by the commerce between bodily organs and objects….These common terms are, in fact, the meanings of common names—what Plato calls ‘Forms’ or ‘Ideas’….the judgments involving them are made by the mind, thinking by itself, without any special bodily organ…Any statement we can make about the objects of perception, and therefore any truth, must contain at least one of these common terms. Therefore all knowledge of truths, as distinct from immediate acquaintance with sense-data, involves acquaintance with Forms, which are not private objects of perception, not individual existents, not involved in the Heracleitean flux. The reader can now draw the first conclusion: Perception is not the whole of knowledge.”
Nielsen presents a helpful formalization of Plato’s Wooden Horse Argument:
(1) The five senses—that through which we perceive properties such as ‘hot’, ‘hard’, ‘white’, ‘sweet’ and their opposites—are instruments of the body.
(2) We can’t perceive the proper objects of one sense through another sense.
(3) If we think something about both a proper object of sight and a proper object of taste (that this white thing is sweet, that white differs from sweet, and so on), this is not in virtue of a perception of both properties through either sense.
(4) But we do think such things about particular sights, tastes, and the like.
(5) So whenever we have a thought that includes reference to a ‘common property’, this is not ultimately in virtue of any of the five senses, which are instruments of the body.
(6) So common properties aren’t sensible.
(7) Rather, the soul considers the common properties through itself.
(8) The soul is ‘that one form’ with which we consider the proper objects that are given through the five senses.
If this argument is persuasive, then the soul investigates by itself when it encounters these general concepts or, in Plato’s language, Forms. Now, for Plato the Forms were immaterial. So does the argument imply that the soul is immaterial like the Forms? Well, at Theaetetus 185d, Socrates asks through which part of the body common terms are thought; Theaetetus admits he cannot come up with any bodily organ. So the inference is drawn that thinking must be occurring through something immaterial. Of course, this inability to find some bodily means to think the Forms is not the strongest reason to draw the conclusion that the soul is immaterial. Certainly the dubious principle “like is known by like”, a principle Plato appears to have accepted, could be applied here for a justification. But a more persuasive account would be preferable.
We can turn to Aristotle for this account. Aristotle argued that universals, like existence, difference, sameness, and so on, cannot be located in space and time and therefore must be thought with an immaterial mind. Caleb Cohoe explains:
“Aristotle holds that the sort of cognition we get from cognitive powers with bodily organs is always spatiotemporally limited. In contrast, understanding is universal, though it also applies to particulars. When we understand something we grasp the being of the form itself, not just a particular instance of that form. Since understanding is not spatiotemporally limited, while cognition from a bodily organ or combination of bodily organs is, the intellect cannot have a bodily organ. Aristotle holds that in understanding something we grasp what it is to be a thing of this sort. Merely combining individual perceptible qualities and quantities will not result in such understanding. To understand what a triangle is involves something further beyond seeing a bunch of shapes that happen to be triangles: it involves seeing what a triangle is and why all these shapes are triangles. If our understanding was not of triangle as such, but of triangle at a particular place and time, it would not be applicable to triangles at other places and times. Aristotle denies that we can get this universal sort of understanding from cognitive powers with bodily organs.”
James Royce, in his book Man and His Nature, presents a helpful elaboration that brings in the notion of the soul’s simplicity: “My concept of a triangle cannot be measured. I can have larger and smaller images, but the idea of what a triangle is applies equally well to all triangles, large and small, which could not be possible if the idea had size itself. My idea of an elephant is no bigger than my idea of a flea, for neither is quantified. Ideas do not occupy space: not having parts, they cannot extend over quantified matter. Nor does a simple idea occupy many parts of the brain at once, for then we would have many ideas, not one, of any one thing. A judgment means recognition of identity or non-identity of two concepts; but if one concept is one space, and the other in another part, I could never get the two together in a judgment. The only conclusion is that the ultimate subject of such simple operations is itself simple.”
If this line of argument works then we would have a stronger justification for Theaetetus’ and Socrates’ commitment to the view that the soul, that unity that we sense with, is indeed immaterial. And this would allow us to go further and argue that an immaterial intellect’s ability to think universals supports the possibility that the soul can exist after the body perishes. For if the soul can function apart from matter then perhaps the soul can exist or have its being separate from matter. Royce presents the argument as follows: “If the human soul is intrinsically independent of matter for at least some of its operations, then it must be intrinsically independent of matter in its being, for its being is known by its operations. An effect cannot be above its cause; spiritual operations [i.e., those operations intrinsically independent of matter] can never flow from a material principle”. Royce then connects this insight to the soul’s immortality:
“The human soul is not subject to intrinsic corruption. It cannot corrupt itself (per se), for it is simple and inextended. Not being composed, it cannot break up into parts because it has none. It is not subject to incidental corruption (per accidens), because it is spiritual and intrinsically independent of matter….Not depending wholly upon the body for its rational operations and therefore not for its being, it will not cease to exist when the body dies. The human soul is thus immortal by nature.”
To be sure, there are plenty of controversial ideas here. We could argue, contrary to premise two of the Wooden Horse Argument, that we can perceive the proper objects of one sense through another sense. We could deny there are universals, like nominalists do, and/or we could argue that everything is material like materialists do. We could also agree with those philosophers and scientists who deny we need some one unified place where all sense data is unified. Could it be that, despite the power of the above argument, that unified experience does not require a unity to do the unifying but some distributed set of functions? And isn’t it plausible to argue, especially in light of everything we know about mental illness and brain damage, that all so-called spiritual operations depend completely on a set of brain functions? And even if the mind could function separately for some of its more abstract functions, does that get us to the kind of individual immortality most people think when they hope for an afterlife? And what about the possibility that a simple soul existing independently of the body might have its powers run down to the extent that they are almost non-existent? These are some of the formidable objections advocates of the argument must face.
 K.M. Nielsen, “Did Plato Articulate the Achilles Argument?” in The Achilles of Rational Psychology, eds. Lennon and Stainton (Springer Publishing, 2008), p. 34.
 Francis Cornford, Plato’s Theory of Knowledge (New York: Bobbs-Merrill, 1957), p. 105.
 See his essay “Why the Intellect Cannot Have a Bodily Organ: De Anima III 4”.
 James E. Royce, Man and His Nature (New York: McGraw-Hill, 1961), p. 313. Royce also notes that a condition for the possibility of self-reflection is that the mind is immaterial: “Another proof of the immateriality of the intellect is the fact that it is able to reflect or bend back perfectly upon itself. I can think about my thinking. No extended material being can bend back perfectly upon itself but only upon some other part or power at best; for instance, the central sense can know the activity of the special senses. But the intellect can know itself and its own act by a process of proper and perfect reflection which is beyond the limitations of a material being.” (p. 94). This argument was favored and thoroughly developed by Proclus in his The Elements of Theology (5th century), for example sections 15, 16, 43, 46, 49, 81, 82, 83, 186, and 187.
 Ibid., 314.
 Ibid., p. 325.