Berkeley’s philosophy was challenged by Scottish philosopher David Hume (17111776). Hume agreed with Berkeley that if there were physical entities, we could have no direct experience of them. But he also noted that we have no direct experience of mind entities either. We feel there ought to be such entities, but where? As Descartes noted, minds do not seem spatially located. I cannot point to my mind in the same way I point to a pencil. When I introspect in search of the mind, I encounter thoughts, feelings, and other perceptions but no separate mind. Even if I did envision something distinct each time I introspected—a luminous sphere, perhaps—that would not be the mind but merely a perception. For Hume, minds do not exist as mental containers or other distinct entities—there are just perceptions. Moreover, he has no room for God in his minimalist ontology.
While Berkeley eliminates physical substance, Hume goes further and eliminates the mind as separate from perceptions. He puts it this way:
The mind is a kind of theatre, where several perceptions successively make their appearance; pass, re-pass, glide away, and mingle in an infinite variety of postures and situations. There is properly no simplicity in it at one time, nor identity in different; whatever natural propensity we may have to imagine that simplicity and identity. The comparison of the theatre must not mislead us. They are the successive perceptions only, that constitute the mind; nor have we the most distant notion of the place where these scenes are represented, or of the materials of which it is composed. (Hume 1978, p. 253)
Hume’s view of the mind as constituted by successive perceptions is called the bundle theory of the mind—the mind is a bundle of perceptions (mental content) in flux.
Hume also challenges our notion of causality. He searches for perceivable causes in nature (necessary connections between events) but fails to isolate them. All he finds are sequences of events. Imagine we are playing billiards and one ball hits another so it moves—one ball causes another to move through impact. However, according to Hume, we perceive only a sequence of events without necessary connections. We expect the billiard ball to move by habit. However, after 5 p.m. tomorrow, or at some other time in the future, all colliding billiard balls might result in white rabbits. If this happened, we would be surprised, but our surprise would stem only from failure of expectation. We are habituated to expect what will happen through past experience, but how could the past tell us about the future? Hume argues we cannot know future events on the basis of past events. The uniformity of nature, on which induction depends, cannot be proven. This problem is known as Hume’s problem of induction. To prove the uniformity of nature, we have observations of past events, but relying on them to prove the uniformity of nature would be induction, which relies on the uniformity of nature. For Hume, neither causality nor induction work as we think; they are simply ideas, emerging from perceived regularities. In Hume’s view, there are no causes in nature and we have no rational reasons to think the future will resemble the past.