Nominalism has three senses:
- A denial of metaphysical universals.
- An emphasis on reducing one’s ontology to a bare minimum, on paring down the supply of fundamental ontological categories.
- A denial of “abstract” entities.
William of Ockham, the name most associated with nominalism, agreed with the first and second senses, and in a lesser way, the third sense. The scientific principle called “Ockham’s razor” (or “Occam’s razor”) focuses on the second sense.
Ockham’s “nominalism,” in both the first and the second of the above senses, is often viewed as derived from a common source: an underlying concern for ontological parsimony. This is summed up in the famous slogan known as “Ockham’s Razor,” often expressed as “Don’t multiply entities beyond necessity.” Although the sentiment is certainly Ockham’s, that particular formulation is nowhere to be found in his texts. Moreover, as usually stated, it is a sentiment that virtually all philosophers, medieval or otherwise, would accept; no one wants a needlessly bloated ontology. The question, of course, is which entities are needed and which are not.
What this means for science is not a vague simplicity but qualitative parsimony:
This distinction is between qualitative parsimony (roughly, the number of types (or kinds) of thing postulated) and quantitative parsimony (roughly, the number of individual things postulated). The default reading of Occam’s Razor in the bulk of the philosophical literature is as a principle of qualitative parsimony.
Scientific nominalism requires that science minimize or eliminate qualitative distinctions. For example, if there is a choice between theory A with two qualities and theory B with one quality (with or without a larger number of entities), then scientific nominalism will lead science to select theory B and reject the qualitative distinction in theory A.
The ultimate scientific theory for scientific nominalism is a theory with only one quality. In physical science this is a theory in which everything is the same stuff – matter or energy or whatever. The existence of any other qualities would be rejected. If this leads to larger quantities, that is an acceptable price of unification.
Are space and time different qualities? Certainly in everyday life there is a qualitative difference between the space of a waiting room and the time spent waiting. But scientific nominalism must reject (or seek to reject if at all possible) this or any qualitative distinction.
Relativity theory unified space and time into 4D spacetime, although space contributed three of the four dimensions and time contributed only one. So there was still a quantitative difference. But if both space and time are three dimensional, then there is no significant difference between them.
However, if we are open to two complementary theories, then a qualitative distinction between space and time may be incorporated. This would be to accept a kind of plurality of theories instead of insisting on unity. The benefit is that the connection between everyday life and scientific theory is preserved.
Unification can take place without denying plurality. If plurality is symmetric, one need not force a unity. There is no need to combine symmetric pluralities. Symmetry is itself a kind of unity within plurality.