“… all knowledge starts from experience and ends in it.
Propositions arrived at by purely logical means are
completely empty as regards reality.”
“Let us ask, does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: For it can contain nothing but sophistry and illusion.”
Evidence is the connection between our beliefs and the real world; it is how information flows and spreads through relationships of cause and effect. When information from the world reaches our senses and arrives in our brains, its patterns are analyzed. In this way our brains build up a representation of the patterns of the world.
To borrow Alfred Korzybski’s famous map/territory metaphor, evidence is what connects the “map” in your head with the “territory” in the real world. The only way to make a true map is to go look at the territory, and draw a map from what you see. If you draw a map using only your imagination, it will not be a useful map because it will not correctly represent the territory. Similarly, if you form a belief that’s not based on something observed in the real world, the belief will not be useful because it will not correctly represent the real world.
A reliable belief – a belief that tends to correspond to the actual state of the world – must be the result of a sensory experience that is causally connected to the corresponding thing in the world. To form a reliable belief, you need to see, hear, smell etc. something that would tend to be different depending on the state of the thing in the world that you want to know about.
The connection between our beliefs and the world is essentially a one-way street; beliefs (ideally) reflect the real world, but the world does not have to conform to our beliefs. There is one sense in which causation can seem to flow the other way: our beliefs affect our thoughts and actions, and our actions in turn affect the world. This is because our brains, in addition to representing the world, are real objects that themselves exist in the real world, just as a map is a real object that exists in the territory. Our beliefs can affect other things in the real world in exactly the same way that everything else does, through ordinary causal processes by informing our actions. If a spot on a map looks like a good place to build a house, you might go build a house there and thereby change the territory, but you can not simply draw a house on a map and then expect it to appear in the territory.
There is an alarming tendency for people to look at the way beliefs correspond to the real world, and then infer the causality in the wrong direction. The idea is widespread in human cultures and rituals that believing or wishing for something can directly cause it to actually happen. Often this fallacy of wishful thinking is harmlessly playful and not taken too seriously, for example, when blowing out candles on a birthday cake, tossing a coin into a well or a fountain, observing a meteoroid or breaking a turkey’s furcula. In other cases it is intended very seriously. Napoleon Hill claimed in his 1937 book Think and Grow Rich that your brain can attract your desires by sending out “vibrations of thought”. More recently, Rhonda Byrne and Oprah Winfrey have promoted the “law of attraction”, the idea that the universe will somehow grant to you whatever you think about consistently. Infomercial pitchman (and convicted fraudster) Kevin Trudeau has even repackaged and sold the idea as a secret long hidden by a shadowy conspiracy. When people join cults and buy into entire belief systems based on this logical fallacy, it is no longer harmless.
A hundred years ago, before fields like information theory and computational neuroscience existed, this kind of thinking may have been more forgivable but today the prevalence of these ideas is inexcusable. We know how information works. We understand how it interacts with the brain. We can build information processing machines ourselves. Today we understand how information works in the brain well enough that we can calculate the amount of information carried by the firing of a neuron and maybe even build a virtual human brain in a supercomputer.
If you fill your car’s tank with fuel, the fuel gauge will read full. If you look at the gauge, your brain can observe that the needle is in the “full” position and you can form a reliable belief that the tank is full. What you can’t do is cause the tank to become full by simply believing it to be full. This would be like gluing the fuel gauge’s needle in the “full” position and then expecting never to run out of gas. Information just doesn’t work that way. The only way your belief about the state of your car`s fuel tank can affect the actual state of your car`s fuel tank is by causing you to take an action like adding more fuel to the tank.
Incidentally, anyone who thinks that beliefs cause events in the world and not the other way around can easily test this belief by having someone hit them over the head with a baseball bat while believing the bat is made of foam.