It is true that Reason must be supplied with First Principles, whence to take its start; and these First Principles are here declared to be, fixed emotional states or dispositions, engendered in the mind of the agent by a succession of similar acts.

But even these dispositions themselves, though not belonging to the department of Reason, are not exempt from the challenge and scrutiny of Reason; while the proper application of them in act to the complicated realities of life, is the work of Reason altogether.

Such an ethical theory calls upon Aristotle to indicate, more or less fully, those intellectual excellences, whereby alone we are enabled to overcome the inherent difficulties of right ethical conduct; and he indicates them in the present Book, comparing them with those other intellectual excellences which guide our theoretical investigations, where conduct is not directly concerned.
In specifying the ethical excellences, or excellences of disposition, we explained that each of them aimed to realize a mean--and that this mean was to be determined by Right Reason.

To find the mean, is thus an operation of the Intellect; and we have now to explain what the right performance of it is,--or to enter upon the Excellences of the Intellect.

The soul having been divided into Irrational and Rational, the Rational must farther be divided into two parts,--the Scientific (dealing with necessary matter), the Calculative, or Deliberative (dealing with contingent matter).