Question: "What is the principle of sufficient reason?"
Answer: The principle of sufficient reason is closely related to cosmological arguments for the existence of God. Its name is somewhat confusing. In this phrase, reason has been used to either mean “explanation” or “cause,” and these two definitions lead to drastically different conclusions. The principle of sufficient reason can be generally stated as “every fact, entity, or occurrence has an adequate explanation for why it is true, exists, or happens.” Or, more simply, “there is a reason for everything.”
In general philosophy, the principle of sufficient reason generates vast resources for discussion. Debates continue over how to determine when an explanation is sufficient and whether certain facts must be accepted without supporting reasons. The principle is also used as a proverb to encourage rational thinking, by forbidding “just because” answers. As with many deep ideas, how one interprets the principle of sufficient reason depends on how the terms are defined and what other worldview principles are in play.
So far as it relates to God, the principle of sufficient reason is intertwined with cosmological arguments. In that category, defining the term reason becomes especially crucial. If the axiom is stated as “everything has a cause,” it leads to a logical paradox. Phrasing it as “everything has an explanation” removes that problem.
Tying the principle of sufficient reason to causality—saying “everything that exists has a cause”—leads to a logical paradox. That arrangement implies a never-ending chain of causality. At no point can there be a “beginning,” since every step in the process must have its own cause. This is not only physically impossible, but it is logically impossible: there cannot be a literally infinite past. If you can measure or traverse the time between points A and B, they are not “infinitely” far apart, which means there can be no point in past time “infinitely” before now.
In short, logic demands an un-caused starting point for existence and causality. “Everything has a cause,” as stated, cannot be absolutely true. This is where the kalam cosmological argument comes in. Logic demands something un-caused and eternally existing in order to cause all other things. The universe itself is clearly not that thing. A collection of effects cannot combine into an un-caused cause any more than a collection of black bricks can combine to become a white wall. If the principle of sufficient reason is upheld as “everything has a cause,” then it cannot be true.
Stating the principle of sufficient reason as “everything has an explanation” does not suffer from this flaw. For most things, cause and reason can be interchanged without trouble. When it comes to the logically mandated “First Cause,” there is an “explanation.” That explanation is that there must be something—one thing or One Being—that simply and necessarily exists and that causes all other things to exist. God’s reason for existence is in Himself; this is nothing like saying He is self-caused; rather, it is to say He is un-caused.
This idea is reflected in God’s self-identification to Moses, when He calls Himself “I Am” (Exodus 3:14). God’s existence simply “is” and must be. God is the one and only thing that must exist and has always existed. The principle of sufficient reason, stated correctly, is compatible with this claim. Stated incorrectly, the principle of sufficient reason is both unbiblical and self-defeating.