# How does one know if X is evidence of Y?

How does one know if X is evidence of Y? If one cannot think of a reason as to why X implies Y, is this the same as saying that X is evidence of Y?

For example, clearly, if I see a plane, that isn’t evidence that someone in my house got murdered. They are completely unrelated things.

But is this purely because I cannot think of a reason as to WHY the plane doesn’t imply murder?

• You need a possible chain of cause and effect. Jul 30, 2023 at 10:16
• screaming and someone running away covered in blood is evidence for a murder, and like you say a plane overhead isn't. are you asking what makes something evidence and what are good reasons for citing something as evidence? the A team have a plane and they kill lots of people, but still...
– user66760
Jul 30, 2023 at 11:57
• There is no simple answer. This is a major part of epistemology. Some philosophers have attempted to develop a formal approach to epistemology ( plato.stanford.edu/entries/formal-epistemology ) though there is no general consensus on whether this is feasible. It is also closely related to confirmation theory. ( plato.stanford.edu/entries/confirmation ). Also much of statistical inference bears on it. Jul 30, 2023 at 13:52
• It should be noted that X directly precedes Y, and Y immediately follows X. This is pretty convincing of something, but correlation is not causation. Jul 30, 2023 at 14:24
• If Y could be shown to be true via means not involving X, would that prove or contradict the notion that "X is evidence of Y"? Jul 30, 2023 at 18:27

The relationship between X and Y as evidence and outcome isn't just about whether we can personally think of a connection, it's about whether there is a logical, empirical, or probabilistic connection that can be established. Let's think about this through three different lenses: Deductive reasoning, Inductive reasoning, and Abductive reasoning.

1. Deductive reasoning: In this form of reasoning, if the premises are true, the conclusion must be true. For instance, if we have the premises
• All men are mortal.
• Socrates is a man.

We can deduce the conclusion:

• Socrates is mortal.

Here, the premises are evidence for the conclusion. If you can't find a logical structure that connects X and Y in this way, then X is not deductive evidence for Y.

1. Inductive reasoning: This is where we generalize from specific observations to broader truths. For instance, if we observe that the sun has risen every day of our lives, we might inductively infer that the sun will rise tomorrow. If X is an observation that fits into a pattern that leads us to anticipate Y, then X is inductive evidence for Y.

2. Abductive reasoning: This is often referred to as "inference to the best explanation." We use this kind of reasoning when we have some data and we're trying to figure out the best explanation for that data. For instance, if you wake up and see snow on the ground (X), you might reasonably infer that it snowed last night (Y). Here, X is evidence for Y because Y is the best explanation we have for X.

Now, let's consider your example about seeing a plane and someone in your house getting murdered. Generally, we would say that these things are unrelated because there's no commonly accepted logical, empirical, or probabilistic structure that links them. It's not about you personally being unable to think of a connection; it's that there's no widely recognized reason to think there is a connection.

However, in some very specific contexts, seeing a plane could be evidence of a murder in your house. For example, if there was a known serial killer who always flew a distinctive plane over the houses where he committed murders, then seeing that plane might indeed be evidence of a murder in your house.

So, when you're asking whether X is evidence for Y, you're essentially asking whether there's a logical, empirical, or probabilistic structure that connects X and Y. If there isn't, then X is not evidence for Y.

• I guess the earnest wish of humanity is to somehow come up with deductive reasoning for everything in the universe. But, what fun would that be? Jul 30, 2023 at 15:25