Utterance may be either perfect or imperfect. It is perfect when it contains a proposition; imperfect when the proposition is incomplete. [169] The portion of logic, therefore, which treats of utterance falls into two parts, devoted respectively to the consideration of complete and incomplete expression. [(2) Incomplete expression.] [(a) The grammar of words.] In the section devoted to incomplete expression, much is found which we should include under grammar rather than under logic. Thus all incomplete expressions are divided into two groups—one group includes proper names and adjectives, the other includes verbs. [170] These two groups are used respectively to express what is essential and what is accidental, [171] and are again divided into a number of subdivisions and varieties. [172] To this part of logic investigations into the formation and division of conceptions, and the doctrine of the categories, properly belong; but it cannot be said with certainty what place they occupy in the logic of the Stoics. [173] Certain it is that these researches introduced little new matter. All that is known of the Stoic views in reference to the formation, the mutual relation and the analysis of conceptions, differs only from the corresponding parts in the teaching of Aristotle by the change of a few expressions, and a slightly altered order of treatment. [174] [(b) The Stoic Categories.] Of greater importance is the Stoic doctrine of the categories. [175] In this branch of logic, the Stoics again follow Aristotle, but not without deviating from him in three points. Aristotle referred his categories to no higher conception, but looked upon them severally as the highest class-conceptions; the Stoics referred them all to one higher conception. Aristotle enumerated ten categories; the Stoics thought that they could do with four, [176] which four only partially coincide with those of Aristotle. Aristotle placed the categories side by side, as co-ordinate, so that no object could come under a second category in the same respect in which it came under the first one; [177] the Stoics placed them one under the other, as subordinate, so that every preceding category is more accurately determined by the next succeeding one. [(α) Highest Conception—an indefinite Something.]