Hume’s Uniformity of Nature

The 18th century philosopher David Hume raised issue with inductive logic, which has fundamental ramifications for science. His contention has not been firmly settled, and it is very difficult to foresee a solution to what is now known as “Hume’s Problem” in the near future.

Just as a little bit of background, here is an example of inductive reasoning:

All bodies that have been observed obey Newton’s law of gravitation.

Therefore all bodies obey Newton’s law of gravitation.

We are extrapolating from a small data set (on a cosmic scale) to formulating laws about the universe. Obviously, this reasoning is not perfectly sound, but this is how physicists (and all other scientists) reason. Importantly, there is no sound deductive argument to suggest that all bodies should necessarily obey Newton’s law of gravitation.

Hume calls this assumption the assumption of the Uniformity of Nature (UN). The assertion is that the laws of physics do not change, for example, from object to object.

Only under the assumption of UN does reasoning inductively actually work. However, the UN assumption cannot be proven deductively (at least not yet!), and therefore there is no reason to suppose that the laws of physics might not look totally different, say, in another part of the universe that we have yet to observe.

I think that every practicing physicist is at some level aware of the UN assumption, but it is always illuminating to have ideas explicitly stated. But now that we have articulated it and are plainly aware of its constraints on our reasoning, let us get on with physics.

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