Traditional theoretical physics typically only describes phenomena, rather than investigating mechanisms, or examines surface mechanisms without deeply investigating causes. This paper attempts to investigate the causal level of physics, to discuss a kind of First Mechanics—a foundational system that takes quantity of occupation and its substitution motion as its object of study, explaining phenomena rather than merely describing them, and constructs a corresponding formal mathematical system—in order to supplement the foundational theories of traditional physics. From this, the topic of dynamicity is discovered. Finally, from the study of Quantity of Occupation, spatial dimensionality is discovered, and from spatial dimensionality and unknown phenomena, the Unsolvability Principle is discovered: namely, that we are uncertain about everything. After the main text, a hypothesis explaining dynamicity is appended.