Hume's Functionalism About Mental Kinds Jason Zarri 1. Introduction A very common view of Hume's distinction between impressions and ideas is that it is based on their intrinsic properties; specifically, their force and vivacity. Some interpreters have challenged this, one being David Landy (Landy 2006). He argues that for Hume the difference lies instead in the fact that impressions are not copies of anything, while ideas are copies of impressions. I regard this view as unsatisfactory, not because it is fundamentally mistaken but because (to put it in Humean terms) it "...it discovers not all the truth" (Treatise 1.3.7.4). I will argue that Hume was a functionalist about (some) mental kinds, individuating impressions, ideas, and beliefs (and possibly other mental phenomena) in terms of their causal role in our mental economy. The distinction between impressions and ideas involves the fact that ideas are copies and impressions are not, but also more than that. I will also argue that interpreting Hume as a functionalist enables one to make sense of a passage that is impossible to explain on the force-and-vivacity view, and that it does so more readily than Landy's view. Furthermore, I think this interpretation makes better sense of Hume's "missing shade of blue" than Landy's does. 2. Hume's Views of Impressions, Ideas and Beliefs It is commonly held that Hume thought that what makes impressions and ideas different from each other it that impressions are more forceful, lively, and/or vivacious than our ideas. Hume seems to say as much in the opening section of the Treatise: All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS. The difference betwixt these consists in the degrees of force and liveliness, with which they strike upon the mind, and make their way into our thought or consciousness. (Treatise 1.1.1.1, my emphasis) However, almost as soon as he has said this Hume seems to take it back: Every one of himself will readily perceive the difference betwixt feeling and thinking. The common degrees of these are easily distinguished; though it is not impossible but in particular instances they may very nearly approach to each other. Thus in sleep, in a fever, in madness, or in any very violent emotions of soul, our ideas may approach to our impressions, as on the other hand it sometimes happens, that our impressions are so faint and low, that we cannot distinguish them from our ideas. (Treatise 1.1.1.1, my emphasis, footnote omitted.) As Landy has pointed out (Landy 2006, p. 121), Hume can hardly be convinced that the difference between impressions and ideas consists in their different degrees of force and liveliness, in any strong sense of 'consists', if they can be so similar that we cannot tell them apart. Though these different degrees of force and liveliness may be very reliable indicators of the difference between impressions and ideas, the criterion for what makes them distinct must lie elsewhere. Where could that be? I think that Hume tells us, a little later in the Treatise in the course of expounding his account of belief: An idea assented to FEELS different from a fictitious idea, that the fancy alone presents to us: And this different feeling I endeavour to explain by calling it a superior force, or vivacity, or solidity, or FIRMNESS, or steadiness. This variety of terms, which may seem so unphilosophical, is intended only to express that act of the mind, which renders realities more present to us than fictions, causes them to weigh more in the thought, and gives them a superior influence on the passions and imagination. [...] And in philosophy we can go no farther, than assert, that [belief] is something felt by the mind, which distinguishes the ideas of the judgment from the fictions of the imagination. It gives them more force and influence; makes them appear of greater importance; infixes them in the mind; and renders them the governing principles of all our actions. (Treatise 1.3.7.7, my emphases.) Note well what Hume says in the italicized phrases: He says that assenting to an idea causes realities to "...weigh more in our thought, and gives them a superior influence on the passions and imagination," and moreover, he calls the way than an assented-to idea feels an act of mind. Furthermore, he says that belief gives 'ideas of judgment' more influence, and "...renders them the governing principles of all our actions." These are all functional properties; they form a part of the causal role that assent or belief plays in our mind, and though connected with the intrinsic properties of the perceptions that are the beliefs themselves, they remain distinct from them. The same, I suggest, is true of the distinction between impressions and ideas: What distinguishes them is not their different degrees of force and vivacity, but rather the different parts they play in our mental lives. Like Landy, I believe that it is part of the causal role of impressions that they are not copies of anything, while it is part of the causal role of ideas that they are. But unlike him, I think that their causal roles also include more besides. In particular, I think that it is part of the causal role of impressions that they give rise to belief and thereby influence behavior, while it is part of the causal role of ideas that that they do not give rise to belief or influence behavior-at least not qua ideas. The issue of whether and in what sense they can will be the subject of the next section. 3. The Influence of an Idea? In Section 8 of Part 3 of Book 1, Hume voices an objection to his account of belief which he immediately proceeds to answer: ... my general position, that an opinion or belief is nothing but a strong and lively idea derived from a present impression related to it, maybe liable to the following objection, by reason of a little ambiguity in those words strong and lively. It may be said, that not only an impression may give rise to reasoning, but that an idea may also have the same influence; especially upon my principle, that all our ideas are derived from correspondent impressions. For suppose I form at present an idea, of which I have forgot the correspondent impression, I am able to conclude from this idea, that such an impression did once exist; and as this conclusion is attended with belief, it may be asked, from whence are the qualities of force and vivacity derived, which constitute this belief? And to this I answer very readily, from the present idea. For as this idea is not here considered, as the representation of any absent object, but as a real perception in the mind, of which we are intimately conscious, it must be able to bestow on whatever is related to it the same quality, call it firmness, or solidity, or force, or vivacity, with which the mind reflects upon it, and is assured of its present existence. The idea here supplies the place of an impression, and is entirely the same, so far as regards our present purpose. [...] After this any one will understand how we may form the idea of an impression and of an idea, and how we way believe the existence of an impression and of an idea. (Treatise 1.3.8.15-7, my emphases.) This answer ought to strike defenders of the force-and-vivacity interpretation as very puzzling. If impressions and ideas are distinguished solely by their different degrees of force and vivacity, how is it that an idea can "...suppl[y] the place of an impression, and [be] entirely the same, so far as regards [Hume's] present purpose"? If an idea can do that, it must have the same degree of force and vivacity as an impression, and hence be an impression. It also accounts for this passage more readily than Landy's interpretation: For him, it is essential to ideas that they are copies and impressions that they are not, for that is what makes them distinct. But here Hume seems to equate them. Landy could object that Hume only means equate them in terms of their influence on belief. This may be true, but it is not the whole truth. If we ask what grounds this equation, the answer is that it is characteristic of impressions that they have an influence on our belief. That seems to fit better with my interpretation, for on my view having this influence on belief is one of the criteria that defines what it is to be an impression; on his view, it is only a symptom. My interpretation, on the other hand, can easily account for this passage. On my view, the causal roles of impressions and ideas are distinct because they are different functions that mental items can have. That allows for the possibility that, though these roles are distinct, one and the same mental item can have both functions. Consider an analogous case: Being a computer is a functional kind, because whether something is a computer depends only on what kinds of operations it can perform. So is being a paperweight. These are distinct functions, for they are characterized by different causal roles, and yet one and the same thing can play both-computers can make good paperweights, even though they aren't typically used that way. Similarly, the same mental item, which normally serves as an idea, and which has the low degree of force and vivacity characteristic of an idea, can also play the role of an impression. I will turn next to the passage which poses the most difficult problem for Landy's view: Hume's (in)famous "missing shade of blue". 4. An Idea without an Impression? In Treatise 1.1.1.10, Hume gives a counterexample to his copy principle, the "missing shade of blue," which he proceeds to dismiss as so exceptional that we should not abandon the copy principle for its sake alone. This ought to be impossible on Landy's interpretation, for according to it the criterion of something's being an impression is that it be copied from an impression, while for Hume this particular purported idea is not. Landy tries to account for this by saying that, though the idea was not copied from an impression, it is "as good as" copied (Landy 2006, pp. 133-4). However, that should only make it as good as an idea, not an idea itself. There wouldn't be a problem here if there were other criteria that one could resort to to help individuate ideas, bet their being copies of impressions is the only one that Landy explicitly supplies. On my account, however, one can also individuate ideas in terms of the kind of influence they have on other mental phenomena. One again, Landy's account is not wrong per se, but I think that mine offers a more complete picture of the situation. 5. Conclusion I have argued that Hume is a functionalist about certain mental kinds. This view makes better sense of what Hume says about impressions, ideas, and beliefs than the force-and-vivacity interpretation, and offers a more complete view than Landy's interpretation. Though he was not a functionalist about qualia themselves, Hume's functionalism about mental kinds makes him an ancestor of contemporary functionalists, just one of the ways in which Hume was a philosopher ahead of his time. References Hume, David. A Treatise of Human Nature, Edited by David Fate Norton and Mary J. Norton. Oxford University Press, Oxford 2001. Landy, David. "Hume's Impression/Idea Distinction." Hume Studies Volume 32, Number 1, (2006) 119-140.