If you had a dollar for every time you heard the phrase ‘scientific proof’, how rich would you be? Scientific proof is one of the surest ways we can know about the world around us; however, pure scientific proof is rare because it is a statement of causality: that Cause C, and only Cause C, created Result R. A scientific experiment attempts to examine all the ways that Result R may happen, by keeping the environment the same and changing one aspect at a time, to see which of the possible causes is the actual one.

The problem is that there may be aspects of the environment we can’t control that affect the experiment. We can’t control gravity for example, or we have to make the experiment happen, observe the process and examine the results. We can’t be sure how far these uncontrolled factors affect the results. Even if the connection between cause and result seems beyond all reasonable doubt, for example that gravity causes all objects to fall to the ground, we might be guilty of creating that connection by logical inference. While such an inference is, for all practical purposes, rational and amply justified, logical inference isn’t scientific proof.

For example, if there were no gravity, the centrifugal effect of Earth’s spin would hurl us all off the planet. We wouldn’t float off, because the Earth is rotating on its axis at 1,440 kilometres (900 miles) per hour. Since we don’t, we infer that the pulling power of Earth’s gravity is stronger than the centrifugal force generated by the Earth’s rotation. Having made that inference – gravity is stronger than rotation, we test it using the scientific method – quantify, measure and repeat.

But what we measure is only the effect of gravity. We can’t measure gravity itself because gravity is a force. Like any force, gravity is a description of one of the ways that two objects relate to one another (“Gonna dig me a hole/ gonna put a nerd in it”). We can only measure the effect a force has on tangible objects. We describe the effect of a force as its power – the greater the effect, the more powerful the force – and we quantify power by how much the force affects those objects.

The result of all this is that we don’t scientifically know what causes things to fall to the ground when we drop them; however, from what we see we infer the existence of a force we call gravity. We can’t prove that gravity exists because we can’t measure gravity itself; we can only measure how gravity affects things. Logically, there could be more than one force operating, but we don’t know: all we see is a single effect. This means that we can’t scientifically prove that gravity exists! But to deny the existence of gravity, until we have another contender with far greater evidence to take its place, would gain its proponents only ridicule.

To achieve pure scientific proof, inferences – no matter how self-evident or logically correct – cannot fill whatever gaps there may be; but for most practical purposes, these inferences are acceptable. In the case of gravity, we can’t scientifically prove what makes a nerd fall into a hole, but in the absence of evidence to the contrary we use Occam’s razor: don’t include more factors than necessary. Gravity and Nelson Muntz are sufficient to explain the phenomenon.

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