KNOWLEDGE when we have knowledge, and look for some criterion to distinguish knowledge from mere belief; without this, they think, we can never achieve certainty But this, Spinoza says, is to begin at the wrong end In order to know that we know, we must Wrst know; and in order to achieve certainty we need no special sign beyond the possession of an adequate idea He who has a true idea knows eo ipso that he has a true idea, and cannot doubt its truth (Eth, 58) ‘How can a man be sure that his idea corresponds to its object?’ philosophers ask Spinoza replies: ‘His knowledge arises simply from his having an idea that does in fact correspond to its object; in other words, truth is its own criterion’ (Eth, 59) The diVerent stages of knowledge correspond to ideas with diVerent properties An idea may be true without being adequate, and it may be adequate without being clear and distinct From the experience of our body coming into contact with other objects, we gather not only ideas of individuals like Peter but also general ideas such as man, horse, or dog Spinoza explains the origin of such general ideas in the following manner: They arise from the fact that so many images, for instance, of men are formed simultaneously that they overpower the faculty of imagination—not entirely, but to the extent that the mind loses count of small diVerences between individuals (colour, size, and so on) and of their actual number It imagines distinctly only that which the individuals have in common in so far as the body is aVected by them—for that is the point in which each of the individuals principally aVected it—and this the mind expresses by the name man and it predicates it of inWnite individuals (E II.112) Other ideas are formed from symbols, from our having read or heard certain words These ideas, while they are true, are confused and unsystematic Our repertoire of such notions constitutes our knowledge of the Wrst kind, which we may call ‘opinion’ or ‘imagination’ There are some ideas, however, which are common to all human beings, which represent adequately the properties of things Such are the ideas of extension and motion Spinoza deWnes an adequate idea as ‘an idea which, insofar as it is considered in itself, without relation to the object, has all the properties or intrinsic marks of a true idea’ (Eth, 32) It is not quite clear how this is to be reconciled with his statement that a true idea needs no mark of its truth It is tempting to think that Spinoza means merely that adequate ideas express truths that are self-evident and are not derived by deduction from other truths But in fact adequate ideas are linked by logical 140