The "law of probability" first appears Aristotle's Poetics in section I, part 7:
And to define the matter roughly, we may say that the proper magnitude is comprised within such limits, that the sequence of events, according to the law of probability [italics added] or necessity, will admit of a change from bad fortune to good, or from good fortune to bad.
Here, "the law of probability" is defined by the action within a text (all literary texts) moving from "bad fortune to good"—or the opposite, "good fortune to bad." As readers, we expect something to happen over the course of a text. Conflict must happen and be resolved. A character must grow. A lesson should be learned. Because this tends to happen in a vast number of texts, the probability of it happening in any one text being read at that moment should be no different.
Aristotle goes on to define "the law of probability" as something which can also be called "universal" (part IX). For example, when referencing characters, Aristotle states, "By the universal I mean how a person of a certain type on occasion speak or act, according to the law of probability or necessity." What this means is that a reader must be able to characterize a character based on stereotypes associated with characters they have "met" before in reading. If the character happens to deviate from the expectations the reader has placed on them (the character), the character tends to be problematic for the reader.
Essentially, Aristotle's "law of probability" allows a reader to assume that what tends to happen will continue to happen.