Full Idea
That part of the soul which we call rational is desiderative: …it desires truth, knowledge, learning, understanding, and recollection - in short, all the good things.
Gist of Idea
The rational part of the soul is the desire for truth, understanding and recollection
Source
Galen (The soul's dependence on the body [c.170], Kiv.2.772)
Book Reference
Galen: 'Selected Works of Galen', ed/tr. Singer,P.N. [OUP 1997], p.152
A Reaction
Truth is no surprise, but recollection is. Note the separation of knowledge from understanding. This is a very good characterisation of rationality. For the Greeks it has a moral dimension, of wanting what is good.