HUME: MENTAL MECHANICS
 Hume presents the fundamentals of his “Theory of Ideas” in Enquiry 2—the mental mechan-
ics that support his new science of the mind. He begins with the following three claims:
(T1) All perceptions are either (a) impressions, or (b) ideas.
(T2) Sensing and sentiment produce only impressions, whereas thinking produces only ideas.
(T3) All ideas are ‘copies’ of impressions.
Impressions are distinguished from ideas in being more “forceful and lively” in themselves; it
roughly matches up with the distinction between feeling and thinking, present experience and
non-present experience (e.g. memory). The last of these, (T3), is known as the “copy thesis”
because it links impressions and ideas. Hume offers two arguments for (T3) in §§2.6–7; the ﬁrst
argument depends on a distinction that requires a restatement of both theses put forward so
far. Hume now says that ideas may be either simple or complex. Now ‘simple’ and ‘complex’
are relational terms: simple/complex with respect to, well, what? Hume wheels in a venerable
theory: he claims, on the basis of introspection, that thinking is a matter of combination and
division, and hence that a single idea may be produced by the combination of many ideas. Thus
simplicity and complexity are explained in terms of the composition of ideas. Hume’s example
makes this clear. The idea of a golden mountain, as he says in §2.5, is clearly derived from the
imaginative combination of the idea-of-gold and the idea-of-the-mountain. (It’s worth noting
that Hume assumes that compounding ideas results in an idea with compound content—not an
obvious claim.) Hence we may reformulate Hume’s explicit theses as follows:
(T1*) Each perception is either (a) an impression, or (b) an idea that is either (b1) simple in
itself or (2 ) a complex composite of simple ideas.
(T3*) Each idea is either (a) a simple idea that is a copy of an impression, or (b) a complex
composite of simple ideas, each of which is a copy of an impression.
Note Hume’s phrasing of (T3*)(a): simple ideas are copies of impressions. That is, Hume holds
that, as a matter of fact, we can not only divide mental phenomena exclusively and exhaustively
into two groups, but that one group consists only of copies of members of the other group. Now
Hume offers two arguments for (T3*). The ﬁrst is a general burden-of-proof argument: Hume
announces that all ideas are ultimately copies of impressions, and challenges his opponents to
produce a counterexample. The second argument is based on a (putatively) factual account of
the way human minds work. In the case of those who are (say) blind from birth, and hence
lack the relevant impressions, we ﬁnd that they do not have the corresponding ideas. This
second argument reveals that Hume has adopted another thesis without putting it on the table
for discussion, namely the converse of (T2):
(T4) Impressions are produced only by sensing and sentiment, whereas ide