オッカムの剃刀

必要が無いなら多くのものを定立してはならない。少数の論理でよい場合は多数の論理を定立してはならない。

物事を説明するのに必要以上の仮定をおいてはいけない、という考え方。14世紀の哲学者オッカムによってよく使われたことから、のちに「オッカムの剃刀」と呼ばれるようになる。本人はカミソリとは呼んでいない。

Ernst Mach advocated a version of Occam's razor which he called the Principle of Economy, stating that "Scientists must use the simplest means of arriving at their results and exclude everything not perceived by the senses." Taken to its logical conclusion, this philosophy becomes positivism; the belief that there is no difference between something that exists but is not observable and something that doesn't exist at all. Mach influenced Einstein when he argued that space and time are not absolute but he also applied positivism to molecules. Mach and his followers claimed that molecules were metaphysical because they were too small to detect directly. This was despite the success the molecular theory had in explaining chemical reactions and thermodynamics. It is ironic that while applying the principle of economy to throw out the concept of the ether and an absolute rest frame, Einstein published almost simultaneously a paper on brownian motion which confirmed the reality of molecules and thus dealt a blow against the use of positivism. The moral of this story is that Occam's razor should not be wielded blindly. As Einstein put it in his Autobiographical notes:

"This is an interesting example of the fact that even scholars of audacious spirit and fine instinct can be obstructed in the interpretation of facts by philosophical prejudices."

しかしこれによって「分子などというものは存在しない」という今の科学から見れば誤った結論にたどり着いてしまった。