This paper addresses the question of the relationship between the object of formal logic and the object of ontology. The history of logic and philosophy shows a kinship and overlapping between the two sciences. The analyses were conducted on the basis of three approaches to formal logic, i.e. Aristotle’s logic Rus­sell’s and Whitehead’s logic, and Leśniewski’s logic. At the same time, it sought to grasp its material and formal object. Now with regard to ontology mainly Aristotelian and Leibnizean understanding of (...) ontology was taken into account as an instance of philosophical ontology and set theory ontology as an example of ontology constructed with a view to play the role of a semantic background for the formulas of logical calculi. The considerations under study allow us to state that even if material objects in the two sciences overlap, especially in the case when the formulas of logic are interpreted by way of ontology, their formal objects seem to be different. Logic does not focus on the question of the cate­gorisation of reality; the essences belonging to respective ontological categories are only “truth-making” factors for the statements formulated by formal logic. (shrink)