This leaves open the possibility that one or more material...

This leaves open the possibility that one or more material existents are merely causes for some phenomena and that they themselves are not the effects of other material existents (although they may be the effects of supernatural causes), the possibility also is left open that they are merely effects of material causes, and that they themselves are not causes of other material phenomena.

The third form is: “Every material existent has a material cause,” and the fourth form is: “Every material existent is the cause of another material existent and is the effect of another material existent.” An implication of the third proposition is a backward stretching infinite regress of material causes. An implication of the fourth proposition is an infinite regress in both directions.

Among these propositions, the first is certain and close to being self-evident, and it is the one which may be called innate. However, regarding the other propositions, they have been more or less in dispute and subject to differences of opinion which are presented in detailed philosophical texts under various topics.

Just as the principle of the existence of material things is not self-evident and needs to be proven, the existence of the causal relation among material things is also not self-evident.

The warrant for this belief is not at the same level as the belief in the universal principle of causation, in the form of a verity proposition ( qaḍiyyah ḥaqīqiyyah ), nor is it on the level of the belief in the existence of the causal relation for the totality of existents, some of whose instances are known through knowledge by presence.

Rather, its logical warrant is at the level of speculative certainties ( naẓariyyāt yaqīnī ), which on the one hand are based on the self-evident principle of causality, and on the other hand are based on empirical premises.