In this work the distinguished physical chemist and philosopher, Michael Polanyi, demonstrates that the scientist's personal participation in his knowledge, in both its discovery and its validation, is an indispensable part of science itself. Even in the exact sciences, "knowing" is an art, of which the skill of the knower, guided by his personal committment and his passionate sense of increasing contact with reality, is a logically necessary part. In the biological and social sciences this becomes even more evident.