Bolzano begins his work by explaining what he means by "theory of science", and the relation between our knowledge, truths and sciences. Human knowledge, he states, is made of all truths (or true propositions) that men know or have known. This is, however, only a very small fraction of all the truths that exist, although still too much for one human being to comprehend. Therefore, our knowledge is divided into more accessible parts. Such a collection of truths is what Bolzano calls a science ("Wissenschaft"). It is important to note that not all true propositions of a science have to be known to men; hence, this is how we can make discoveries in a science.