Objection 1. It would seem that the speculative and practical intellects are distinct powers. For the apprehensive and motive are different kinds of powers, as is clear from De Anima ii, 3. But the speculative intellect is merely an apprehensive power; while the practical intellect is a motive power. Therefore they are distinct powers.
Objection 2. Further, the different nature of the object differentiates the power. But the object of the speculative intellect is "truth," and of the practical is "good"; which differ in nature. Therefore the speculative and practical intellect are distinct powers.
Objection 3. Further, in the intellectual part, the practical intellect is compared to the speculative, as the estimative is to the imaginative power in the sensitive part. But the estimative differs from the imaginative, as power form power, as we have said above (Question 78, Article 4). Therefore also the speculative intellect differs from the practical.
On the contrary, The speculative intellect by extension becomes practical (De Anima iii, 10). But one power is not changed into another. Therefore the speculative and practical intellects are not distinct powers.
I answer that, The speculative and practical intellects are not distinct powers. The reason of which is that, as we have said above (Question 77, Article 3), what is accidental to the nature of the object of a power, does not differentiate that power; for it is accidental to a thing colored to be man, or to be great or small; hence all such things are apprehended by the same power of sight. Now, to a thing apprehended by the intellect, it is accidental whether it be directed to operation or not, and according to this the speculative and practical intellects differ. 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.
Reply to Objection 1. The practical intellect is a motive power, not as executing movement, but as directing towards it; and this belongs to it according to its mode of apprehension.
Reply to Objection 2. Truth and good include one another; for truth is something good, otherwise it would not be desirable; and good is something true, otherwise it would not be intelligible. Therefore as the object of the appetite may be something true, as having the aspect of good, for example, when some one desires to know the truth; so the object of the practical intellect is good directed to the operation, and under the aspect of truth. For the practical intellect knows truth, just as the speculative, but it directs the known truth to operation.
Reply to Objection 3. Many differences differentiate the sensitive powers, which do not differentiate the intellectual powers, as we have said above (7, ad 2; 77, 3, ad 4).
Source: Summa Theologica (New Advent)