The Commens Dictionary
Quote from ‘Smithsonian Institution letters’
Deduction is necessary inference, where we hold to the conclusion because we think we see clearly that the premisses could not, in any constitution of the universe be true without that conclusion being true along with them. Deduction relates to an ideal state of things in which its premisses are absolutely true. Although all deduction is necessary inference, it may relate to probabilities. Thus, if I know the probability that any given man will die in December, and know the like for January and February, the sum of those three probabilites is necessarily the probability that he will die in a winter month. Probability is the ratio of the number of individuals in a species to the number of individuals in a genus over that species, within a certain course of experience. Deduction, not making any real observations, only observing our own premised ideas, can never ascertain what a probability really is, but only calculate what it would be supposing certain other probabilities are so and so.