The article is devoted to the justification of the law of inertia. It is often called Newton's first law. It was established that this is not a law, but a postulate. Modern definitions of this law are given. It turned out that well -known definitions of this law are similar to each other. It is shown that this law before Newton was formulated by Descartes, Balillians, Ballo and Galileo. The ontology and philosophical significance of the category "cause" are considered. It is shown that any thing or phenomenon arises for some reason, and nothing happens for no reason and nothing exists. And Democritus understood that there was nothing causeless and could not be. Democritus’s mistake is that he did not recognize a chance. Causality is a philosophical category showing the ontological connection of such two phenomena, when the second phenomenon arises due to the first. Moreover, the first phenomenon is called the cause, and the second is the consequence. In this connection of phenomena, the first phenomenon (cause) necessarily predicts the second phenomenon (investigation). And the mutual relationship between the first and second phenomena is called a causal relationship. is present in all known phenomena of the world, and it is usually understood as the occurrence of the same effects with the same causes. It is shown that in history many ideas and scientific achievements attributed to a certain author have often been discovered before them by other authors. For example, Lobachevsky and Boiai are considered to be the founders of hyperbolic geometry. In fact, this discovery was actually made before them by the now almost forgotten F.A. Taurinus (Taurin). The same can be said about the law of refraction of light. This law was discovered by the Dutch scientist Willebrord Snellius. In fact, 600 years before him, this law had been discovered by the great scientist and philosopher Bahmanyar al-Azerbaijani. And the same mistake is Newton's first law. Therefore, it is best to henceforth always refer to this law as the law of inertia.