For it is the speculative intellect which directs what it apprehends, not to operation, but to the consideration of truth; while the practical intellect is that which directs what it apprehends to operation. And this is what the Philosopher says (De Anima iii, 10); that "the speculative differs from the practical in its end." Whence each is named from its end: the one speculative, the other practical--i.e. operative.Speculation is directed to the contemplation of truth, in contrast to the practical, which is directed to things to be done. It's not a question of mere conjecture at all; rather, its purpose is contemplative, and in particular what distinguishes it from conjecture is that it is contemplation of the truth that is in view for the speculative intellect. Conjecture amounts, maybe, to an opinion, but St. Thomas always maintains a careful distinction between what we believe (i.e., opinions) and what we know (the truth). Truth is known with certainty. We don't guess at it.
This does not mean that such speculation, or contemplation of the truth, is utterly divorced from the practical: "The speculative intellect by extension becomes practical," he writes (ibid).
This contemplation of the truth may be said to be analogous to that vision of God that the saints enjoy in heaven, inasmuch as God himself is truth.