Embryo’s soul: genesis of human soul: Lviv, Printed by the author, 1931, 48 p.

The work presents the author's views on the nature of the soul, in particular on its emergence in the human body. The author argues against the idea that the human soul is single (Latin: simplex) and unchanging, noting that the flaw in this idea lies in the naive conclusion that the soul is indivisible because it is not spatial.  In contrast, the author proposes to consider the soul as a life principle that endows the material body with the characteristics of one or another organic form and is what sustains life in the body. In addition, according to the author, the soul is not static but develops along with the physical body. Kostelnyk substantiates this thesis with the development and complication of the soul in more complex organisms.

The author concludes that the greatest mystery of life in general and the soul as its principle lies in its ability to reproduce itself and be connected with irreproducible matter.