Aristotle defined chance as a cause per accidens, happening for the lesser part. What does this mean?
When a lion chases down a gazelle and kills it, there was a causal relation between the end and the process that led to it. But events don’t come with name tags on them, and so, looking at this happen, we might have said “look, the lion chased down the light-colored thing”. In this case, however, there is no connection between the end (killing a light-colored thing) and the process leading to it. It is accidental, or per accidens. Depending how we account for the experience, the process we experience might be chance or not. But it’s clear that the thing we are experiencing, depending on how it is accounted for, can either be 1.) a process where the end was a cause of the process, or 2.) where the end is not a cause of the process. 2 are events where the end causes per accidens.
There is more than one way to discriminate between 1 and 2. The simplest process is to multiply out experiences and isolate properties. Put a dark-colored gazelle in front of the lion, and then a light colored 747. Experiments to show other things might be more elaborate: for example, it was not known until quite recently that what the frog chased after was not a fly, but a moving object of a certain size. A frog in a room full of dead flies would starve to death. Most persons, now and forever, think that frogs eat flies, and that they eat moving flies only by per accidens (or, that it would even prefer a fly that was not moving). In fact, the reverse is true. Notice that this is the case even though frogs have, in fact. always eaten flies.
But if it is accidental that frogs eat flies, even while it happens that they have always sought them, there is an accidental cause that happens always or as a rule, and another kind of accidental cause that does not happen always or as a rule. Chance events are the second kind. Frogs don’t eat flies per se, but we would not say that they eat them by chance. Chance must be outside both what is per se (frog eats the moving) and what is accidental but occurring always or as a rule (frogs eat flies). Contrariwise, one of the main goals of science- in both the ancient and contemporary senses of the term- is to find a way to rule out both chance and the accidental but occurring always or as a rule. Teh former is frequently difficult to rule out, the latter is, in some cases, impossible to rule out.