§ 169. Generalisation implies abstraction, but we may have abstraction without generalisation. § 170. The following example is useful, as illustrating to the eye how a decrease of extension is accompanied by an increase of intension. At each step of the descent here we visibly tack on a fresh attribute. [Footnote: This example is borrowed from Professor Jevons.] Ship | Steam-ship | Screw steam-ship | Iron screw steam-ship | British iron screw steam-ship. Could we see the classes denoted by the names the pyramid would be exactly inverted. § 171. The law of inverse variation of extension and intension must of course be confined to the inter-relations of a series of terms of which each can be predicated of the other until we arrive at the bottom of the scale. It is not meant to apply to the extension and intension of the same term. The increase of population does not add to the meaning of 'baby.' PART II.--OF PROPOSITIONS. CHAPTER I. _Of the Proposition as distinguished from Other Sentences_. § 172. As in considering the term, we found occasion to distinguish it from words generally, so now, in considering the proposition, it will be well to begin by distinguishing it from other sentences. § 173. Every proposition is a sentence, but every sentence is not a proposition. § 174. The field of logic is far from being conterminous with that of language. Language is the mirror of man's whole nature, whereas logic deals with language only so far as it gives clothing to the products of thought in the narrow sense which we have assigned to that term. Language has materials of every sort lying strewn about, among which the logician has to seek for his proper implements. § 175. Sentences may be employed for a variety of purposes-- (1) To ask a question; (2) To give an order; (3) To express a feeling; (4) To make a statement. These various uses give rise respectively to (1) The Interrogative Sentence; (2) The Imperative Sentence; (3) The Exclamatory Sentence; (4) The Enunciative Sentence; Indicative Potential. It is with the last of these only that logic is concerned. § 176. The proposition, therefore, corresponds to the Indicative and Potential, or Conditional, sentences of grammar. For it must be borne in mind that logic recognises no difference between a statement of fact and a supposition. 'It may rain to-morrow' is as much a proposition as 'It is raining now.' § 177. Leaving the grammatical aspect of the proposition, we must now consider it from the purely logical point of view. § 178. A proposition is a judgement expressed in words; and a judgement is a direct comparison between two concepts. § 179. The same thing may be expressed more briefly by saying that a proposition is a direct comparison between two terms.