In this paper we suggest the use of ontological structures as an appropriate tool for describing the foundations of reality. Every vertex of this structure, representing a fundamental entity in the universe, is completely and solely characterized by its connections to the other vertices in the structure. The edges of this structure are binary compounds of the FEs, and are identified with the elementary particles. The combinations including more than 2 connected vertices correspond to composite particles. The principles according to (...) which the OSs are designed Philosophy of the natural sciences, VHP Tempsky, Vienna, 1989; Shoshani in Phys Essays 4:566–576, 1991; Shoshani in Phys Essays 11:512–520, 1998) are discussed in Sect. 2, and the simplest OSs having the minimal number of vertices, and thus represent the simplest universe, are given in Sect. 3. This section also describes an OS that includes an infinite number of vertices that might represent the space–time points. This structure imparts a new meaning to space–time, detached from their intuitive grasp :285–292, 2010). Section 4 is devoted to show how to ascribe intrinsic properties to the fundamental entities by using their inter-connections in the OS. The predictive power and explanatory capacity of this theory, named Apriorics :126–130, 2014) are briefly described in Sects. 3 and 4. (shrink)