Applying the same principle to mental phenomena, we may determine the law of intellectual action. Thoughts are discriminated by the presence or absence of certain attributes. At one extreme we find the _summum genus_, comprising the fewest possible attributes distinguishing an idea; at the other extreme we find the individual, comprising any number of attributes. Between these two extremes we find a regular series of intermediate terms. The movement of an idea from the general to the individual is like the motion of a planet through one-half of its orbit; while the return movement from the individual to the general, corresponds to the motion of the planet over the remaining half of its orbit. The same law governs both movements and unites the two halves of the orbit into a single whole; and a series of observations taken at equal distances, will, by the uniformity of differences presented, reveal the operation of the same law in this dual manifestation. Upon examining the processes of deduction and induction, we find in each the same series of terms, differing only in the fact that they are in inverse order, and this correspondence reveals the operation of one and the same law. An inductive series is only a deductive series read backward. Any two terms in a series whether inductive or deductive, differ only in the degree of generality, and differ similarly from a third term, so that two being known the third can be therefrom determined. In a deductive series the terms differ by a constant increase in the number of individualizing attributes--a concept being expanded into a deductive series by such regular additions. Having two terms we can proceed to the third--that is, from two propositions expressing this relation, we can proceed to a conclusion. In an inductive series the terms differ by a constant diminution in the number of individualizing attributes--an individual term being expanded into an inductive series, by successively dropping the attributes which compose the individual term, until we reach the required degree of generalization.