8. The establishment of a Proposition requires an attention to observed Facts, and can never be rightly derived from our Conceptions alone. We must hereafter consider the necessity which exists that the Facts should be rightly bound together, as well as that our Conceptions should be clearly employed, in order to {37} lead us to real knowledge. But we may observe here that, in such cases at least as we are now considering, the two processes are co-ordinate. To unfold our Conceptions by the means of Definitions, has never been serviceable to science, except when it has been associated with an immediate _use_ of the Definitions. The endeavour to define a uniform Force was combined with the assertion that 'gravity is a uniform force:' the attempt to define Accelerating Force was immediately followed by the doctrine that 'accelerating forces may be compounded:' the process of defining Momentum was connected with the principle that 'momenta gained and lost are equal:' naturalists would have given in vain the Definition of Species which we have quoted, if they had not also given the 'characters' of species so separated. Definition and Proposition are the two handles of the instrument by which we apprehend truth; the former is of no use without the latter. Definition may be the best mode of explaining our Conception, but that which alone makes it worth while to explain it in any mode, is the opportunity of using it in the expression of Truth. When a Definition is propounded to us as a useful step in knowledge, we are always entitled to ask what Principle it serves to enunciate. If there be no answer to this inquiry, we define and give clearness to our conceptions in vain. While we labour at such a task, we do but light up a vacant room;--we sharpen a knife with which we have nothing to cut;--we take exact aim, while we load our artillery with blank cartridge;--we apply strict rules of grammar to sentences which have no meaning. If, on the other hand, we have under our consideration a proposition probably established, every step which we can make in giving distinctness and exactness to the Terms which this proposition involves, is an important step towards scientific truth. In such cases, any improvement in our Definition is a real advance in the explication of our Conception. The clearness of our impressions casts a light upon the Ideas which we contemplate and convey to others. {38}