The main objective of the paper is to explore how to cognize war and peace. In order to reach this objective, the notions of "war" and "peace" are placed within semantic space of philosophy, which has formal logical, existential, ontological, axiological, praxeological, and, finally, epistemological facets. This approach leads the author to the conclusion that war can be studied by empirical sciences' methods, but peace can be either the subject of philosophical discourses or constructed through "social engineering". For an effective construction of peace, the study of war is necessary.