Suppose knocking over a glass has the direct result of making fall. If you knock over a glass, will it fall, no matter what?

  • "direct result" can be misleading... Sometimes you knock on the glass and the glass will not break. Oct 19, 2021 at 14:14
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    A cause produces an effect, but in "real" cases there are multiple causes co-occurring. Oct 19, 2021 at 14:22
  • The text is not quite the same as the question. If the abstract assumption is If X then Y, then the occurrence of X will indeed produce Y. The discussion has a real example where specific factors, such as the thickness of the glass, introduce uncertainty. I recommend more details, a rephrased question, or both. Nov 19, 2021 at 5:44

3 Answers 3


Rules are logical ideals, not real facts. Rules imply a probability of 100%, except when specified otherwise. But no fact in the universe has a probability of 100%. From a logical standpoint, fall is unavoidable; from an empirical one, probability of 100% is impossible.

In addition, if you knock a glass with a vertical force, it will not even move, normally.

Language is intended to communicate facts in a simple form, and usually rules express normality (that is, 100% of probability in the conditions expected by the observer).

Imagine teaching your kid:

"if you knock the glass with a force greater than F in a vertical position > Y and a direction between D1 and D2, there's a probability of 97% that it falls down; and in such case, depending on the composition, C and the structure S, there's an 83% probability that it breaks"...

In your ideal scenario, knocking the glass will make it fall down always, if "knocking", and "fall down" are applied in a standard and intended way (you knock the glass with enough horizontal force, in a place allowing it to fall down, etc.), and things occur like expected. But in real life, that's not 100% probable.


If you knock over a glass, will it fall, no matter what?

Yes, but that has to do with semantics rather than causation.

By definition, the term knock over (i.e., "to strike to the ground") conveys that the object that was hit has fallen. If short of falling, the object is only hit, not knocked over.


If "unavoidable" means "necessary", then it is exactly what Hume's coined as the problem of induction1.

The mind can always conceive any effect to follow from any cause, and indeed any event to follow upon another: whatever we conceive is possible, at least in a metaphysical sense.

For example:

  • In the past, the event B followed event A.
  • Event B will necessarily follow the event A.

'In the past' serves as observation, whereas 'will' serves as prediction.

So, how do we know that the sun will necessarily rise tomorrow? Hume:

That the Sun will not rise tomorrow is no less intelligible a proposition and implies no more contradiction than the affirmation that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood.

Moreover, take science. Science can provide us with some solid beliefs about unobserved facts, but it is quite another to claim that the methods employed actually do yield knowledge at the very instant.

Many philosophers following empiricist tradition (such as W. V. O. Quine) concluded then that "necessity" is simply not a part of the reality, or that it is a myth. In contrast, modal logic goes against this tendency by introducing the necessity operator □, coming back to the old rationalist tradition.

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