Logic is the theory of Proof, and everything provable can be exhibited as a proposition, propositions alone being objects of belief. Therefore, the import of propositions, that is, the import of predication, must be ascertained. But, as to make a proposition, i.e. to predicate, is to assert one _thing_ of another _thing_, the way to learn the import of predication is, by discovering what are the _things_ signified by names which are capable of being subject or predicate. It was with this object that Aristotle formed his Categories, i.e. an attempted enumeration of all nameable things by the _summa genera_ or highest predicates, one or other of which must, he asserted, be predicable of everything. His, however, is a rude catalogue, without philosophical analysis of the rationale even of familiar distinctions. For instance, his Relation properly includes Action, Passivity, and Local Situation, and also the two categories of Position [Greek: pote] and [Greek: pou], while the difference between [Greek: pou] and [Greek: keisthai] is only verbal, and [Greek: echein] is not a _summum genus_ at all. Besides--only substantives and attributes being there considered--there is no category for sensation and other mental states, since, though these may rightly be placed, so far as they express their relation, if active, to their objects, if passive to their causes, in the Categories of Actio and Passio, the things, viz., the mental states, do not belong there. The absence of a well-defined concrete name answering to the abstract _existence_, is one great obstacle to renewing Aristotle's attempt. The words used for the purpose commonly denote substances only, though attributes and feelings are equally existences. Even _being_ is inadequate, since it denotes only _some_ existences, being used by custom as synonymous with _substance_, both material and spiritual. That is, it is applied to what excites feelings and has attributes, but not to feelings and attributes themselves; and if we called extension, virtue, &c., _beings_, we should be accused of believing in the Platonic self-existing ideas, or Epicurus's sensible forms--in short, of deeming attributes substances. To fill this gap, the abstract, _entity_, was made into a concrete, equivalent to _being_. Yet even _entity_ implies, though not so much as _being_, the notion of substance. In fact, every word originally connoting simply existence, gradually enlarges its connotation to mean _separate_ existence, i.e. existence freed from the condition of belonging to a substance, so as to exclude attributes and feelings. Since, then, all the terms are ambiguous, that among them (and the same principle applies to terms generally) will be employed here which seems on each occasion to be _least_ ambiguous: and terms will be used even in improper senses, when these by familiar association convey the proper meaning.