9. But though _Definition_ may be subservient to a right explication of our conceptions, it is _not essential_ to that process. It is absolutely necessary to every advance in our knowledge, that those by whom such advances are made should possess clearly the conceptions which they employ: but it is by no means necessary that they should unfold these conceptions in the words of a formal Definition. It is easily seen, by examining the course of Galileo's discoveries, that he had a distinct conception of the _Moving Force_ which urges bodies downwards upon an inclined plane, while he still hesitated whether to call it _Momentum_, _Energy_, _Impetus_, or _Force_, and did not venture to offer a Definition of the thing which was the subject of his thoughts. The Conception of _Polarization_ was clear in the minds of many optical speculators, from the time of Huyghens and Newton to that of Young and Fresnel. This Conception we have defined to be 'Opposite properties depending upon opposite positions;' but this notion was, by the discoverers, though constantly assumed and expressed by means of superfluous hypotheses, never clothed in definite language. And in the mean time, it was the custom, among subordinate writers on the same subjects, to say, that the term _Polarization_ had no definite meaning, and was merely an expression of our ignorance. The Definition which was offered by Haüy and others of a _Mineralogical Species_;--'The same elements combined in the same proportions, with the same fundamental form;'--was false, inasmuch as it was incapable of being rigorously applied to any one case; but this defect did not prevent the philosophers who propounded such a Definition from making many valuable additions to mineralogical knowledge, in the way of identifying some species and distinguishing others. The right Conception which they possessed in their minds prevented their being misled by their own very erroneous Definition. The want of any precise Definitions of _Strata_, and _Formations_, and _Epochs_, among geologists, has not prevented the discussions which they have carried on upon such subjects from being highly serviceable {39} in the promotion of geological knowledge. For however much the apparent vagueness of these terms might leave their arguments open to cavil, there was a general understanding prevalent among the most intelligent cultivators of the science, as to what was meant in such expressions; and this common understanding sufficed to determine what evidence should be considered conclusive and what inconclusive, in these inquiries. And thus the distinctness of Conception, which is a real requisite of scientific progress, existed in the minds of the inquirers, although Definitions, which are a partial and accidental evidence of this distinctness, had not yet been hit upon. The Idea had been developed in men's minds, although a clothing of words had not been contrived for it, nor, perhaps, the necessity of