system
Given the following post, respond as if you're commenting on Reddit:
human
What does "A causes B" mean? How can we tell that A causes B?

I am not versed in philosophy, so please try and keep the jargon and the prerequisite knowledge down to a minimum.
gpt
This actually isn't all that simple of a question, as different philosophers have different ideas of what causation means. David Hume, for example, believes that we can never actually know in many cases that A causes B. 

To Hume, there is some types of causation that we can know. These are "relations of ideas", with a common example for him being math. We can always know that 2 + 2 will result in 4, because we can logically show it by showing that there will be a logical contradiction if we assumed that 2 + 2 does not = 4. 

On the other hand, Hume views matters of fact, which are observable things about our world, as being impossible to know causation about. We see a relation through observation between taking an aspirin and it later reducing our headache, but Hume needs a logical way to get from taking an aspirin to reducing a headache every single time in the future.

There are two ways Hume attempts to do this. The first is through "demonstrative reasoning", which is the method we used earlier to show that relations of ideas can be known. However, if we assume that we take an aspirin and we have no headache reduction, there is no logical contradiction so this approach doesn't work. The second method is through "probabilistic reasoning", in which we need to know that **the future will be like the past**. However, then we're stuck with trying to prove that the future will be like the past. This can't be done through demonstrative reasoning because there's no contradiction if we assume that the future will not be like the past, so we must try to show that the future will be like the past through probabilistic reasoning. However, in order to do so, we must already know that the future will be like the past, which to Hume results in circular reasoning.

Thus, Hume believes that it's possible to know causation when it comes to relations of ideas, but not with matters of fact. This is just a small piece of the puzzle, and Kant makes a good effort to flip this on it's head. Hope it helps.