What is the "Law of Causality"?

Question

Is there a Law of Causality? Or a principle of Causality?
Someone used that in an argument with me and I couldn't find much information on the subject. Thank you in advance.

Answer 1

The 'Law of Causality' will most certainly be something similar to:

Every event is an effect of a cause.

Or, even easier:

Every event has a cause.

One example for a canonical formulation of the Law of Causality (as different from the countless empirical laws of causality) can be found in Kant's Critique of Pure Reason:

[...] the [...] natural law of causality that everything contingent must have a cause, [...] (A605/B633, fn., emphasis mine)

Another proposal and very condensed discussion is formulated by Wittgenstein in his (in-)famous Tractatus logico-philosophicus:

6.32 The law of causality is not a law but the form of a law.

6.321 ‘Law of causality’—that is a general name. And just as in mechanics, for example, there are ‘minimum- principles’, such as the law of least action, so too in physics there are causal laws, laws of the causal form.

[...]

6.36 If there were a law of causality, it might be put in the following way: There are laws of nature.

6.361 One might say, using Hertz’s terminology, that only connexions that are subject to law are thinkable.

6.3611 We cannot compare a process with ‘the passage of time’—there is no such thing—but only with another process (such as the working of a chronometer). Hence we can describe the lapse of time only by relying on some other process. Something exactly analogous applies to space: e.g. when people say that neither of two events (which exclude one another) can occur, because there is nothing to cause the one to occur rather than the other, it is really a matter of our being unable to describe one of the two events unless there is some sort of asymmetry to be found. And if such an asymmetry is to be found, we can regard it as the cause of the occurrence of the one and the non-occurrence of the other.

(Translated by D. F. Pears and B. F. McGuinness, Routledge 1974, emphasis mine)

In both cases (Kant as well, but it would take too much place to consider that here) the idea is that in order to perceive something as a particular object/event at all, we must have a difference, an assymetry in space-time, the other. That is our mode of perception, therefore an underlying, necessary principle of (our!) reality. And as both Kant and Wittgenstein point out here, neither time nor space are absolutely objective, just as this "law" is not.

In this sense, it is a principle of the human mode of perception and thought. And therefore very fundamental for each empirical law.