The sentence “snow is white” is true if and only if snow is white.
—Alfred Tarski

To say of what is, that it is, or of what is not, that it is not, is true.
—Aristotle, Metaphysics IV

Walking along the street, your shoelaces come untied. Shortly thereafter, for some odd reason, you start believing your shoelaces are untied. Light leaves the Sun and strikes your shoelaces and bounces off; some photons enter the pupils of your eyes and strike your retina; the energy of the photons triggers neural impulses; the neural impulses are transmitted to the visual-processing areas of the brain; and there the optical information is processed and reconstructed into a 3D model that is recognized as an untied shoelace. There is a sequence of events, a chain of cause and effect, within the world and your brain, by which you end up believing what you believe. The final outcome of the process is a state of mind which mirrors the state of your actual shoelaces.

What is evidence? It is an event entangled, by links of cause and effect, with whatever you want to know about. If the target of your inquiry is your shoelaces, for example, then the light entering your pupils is evidence entangled with your shoelaces. This should not be confused with the technical sense of “entanglement” used in physics—here I’m just talking about “entanglement” in the sense of two things that end up in correlated states because of the links of cause and effect between them.

Not every influence creates the kind of “entanglement” required for evidence. It’s no help to have a machine that beeps when you enter winning lottery numbers, if the machine also beeps when you enter losing lottery numbers. The light reflected from your shoes would not be useful evidence about your shoelaces, if the photons ended up in the same physical state whether your shoelaces were tied or untied.

To say it abstractly: For an event to be evidence about a target of inquiry, it has to happen differently in a way that’s entangled with the different possible states of the target. (To say it technically: There has to be Shannon mutual information between the evidential event and the target of inquiry, relative to your current state of uncertainty about both of them.)

Entanglement can be contagious when processed correctly, which is why you need eyes and a brain. If photons reflect off your shoelaces and hit a rock, the rock won’t ch...

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