The mind in this vocabulary is not a single faculty and it does not have a single significator. It is read from Mercury and the Moon in combination, and the division of labour between them is worth learning, because it is the most useful thing traditional astrology has to say about cognition.
Mercury is the deliberate part. It governs reasoning, speech, writing, calculation, argument, and any skill that has to be trained. It is what a person does with their attention on purpose.
The Moon is the receptive part. It governs impression, memory, imagination, appetite and mood, and it moves faster than anything else in the chart, which is why the tradition uses it for whatever changes on a short cycle. It is what a person's attention does when it is not being directed.
A reading takes both and then takes their relation. Mercury conjunct the Moon describes a mind whose reasoning is saturated with impression. Mercury in aversion to the Moon, meaning in a sign that cannot see the Moon's sign at all, describes one where the two operate without reference to each other. The traditional term for that condition, aversion, is more precise than the modern habit of calling a planet isolated, because it names a specific geometric relation rather than an absence. Both significators are read for condition before they are read for content.