He first of all asked himself what the true value is of our knowledge and what knowledge is. We believe generally that it is the things which give us the knowledge that we have of them. But, rather, is it not we who impose on things the forms of our mind and is not the knowledge that we believe we have of things only the knowledge which we take of the laws of our mind by applying it to things? This is what is most probable. We perceive the things by moulds, so to speak, which are in ourselves and which give them their shapes and they would be shapeless and chaotic were it otherwise. Consequently, it is necessary to distinguish the matter and the form of our knowledge: the matter of the knowledge is the things themselves. The form of our knowledge is ourselves: "Our experimental knowledge is a compound of what we receive from impressions and of what our individual faculty of knowing draws from itself on the occasion of these impressions." SENSIBILITY; UNDERSTANDING; REASON.--Those who believe that all we think proceeds from the senses are therefore wrong; so too are those wrong who believe that all we think proceeds from ourselves. To say, Matter is an appearance, and to say, Ideas are appearances, are equally false doctrines. Now we know by sensibility, by understanding, and by reason. By sensibility we receive the impression of phenomena; by the understanding we impose on these impressions their forms, and link them up together; by reason we give ourselves general ideas of things--universal ones, going beyond or believing they go beyond the data, even when linked up and systematized. Let us analyse sensibility, understanding, and reason. Sensibility already has the forms it imposes on things. These forms are time and space. Time and space are not given us by matter like colour, smell, taste, or sound; they are not perceived by the senses; they are therefore the forms of our sensibility: we can feel only according to time and space, by lodging what we feel in space and time; these are the conditions of sensibility. Phenomena are thus perceived by us under the laws of space and of time. What do they become in us? They are seized by the understanding, which also has its forms, its powers of classification, of arrangement, and of connection. Its forms or powers, or, putting it more exactly, its active forms are, for example, the conception of quantity being always equal: through all phenomena the quantity of substance remains always the same; the conception of causality: everything has a cause and every cause has an effect and it is ever thus. Those are the conditions of our understanding, those without which we do not understand and the forms which within us we impose on all things in order to understand them.