The theoretical approach described in a series of articles (Jarvilehto, 1998a,b,c, 1999, 2000) is developed further in relation to the problems of emotion, consciousness, and brain activity. The approach starts with the claim that many conceptual confusions in psychology are due to the postulate that the organism and the environment are two interacting systems (”Two systems theory”). The gist of the approach is the idea that the organism and environment form a unitary system which is the basis of subjective experience. This starting point leads to the conception of emotions as reorganization of the organism-environment system, and entails that emotion and knowledge are only different aspects of the same process. In the first part of the article the general outline of the approach is sketched, and in a subsequent second part (Jarvilehto, 2001) the relations between emotions, consciousness, and brain activity will be discussed in detail.