Ontological reductionism refers to the belief that the whole of reality consists of a minimal number of entities or substances . However, one might well be trying to reduce material things to some other substance, like consciousness . Alternatively, one might even think that there are two or more irreducible substances. The aim would then be to reduce all other substances to these fundamental few.
Methodological reductionism claims that, in science, 'small is beautiful'.
Thus the best scientific strategy is always to attempt explanation in terms of ever more minute entities. It has undoubtedly been the mark of some of science's greatest successes, and not just in physics. Theory reductionism raises the question of the relation between successive theories in a field, as between Newton's theory and that of Einstein. This kind of thinking has been strongly challenge by such thinkers as the philosopher-historian Thomas Kuhn, who believes that because the terms between theories are always 'incommensurable', theory reduction is never possible. Since this view of reduction is tied strongly to the picture of scientific theories as hypothetico-deductive systems, and since this latter picture has now fallen very much out of favour, many philosophers today would agree with their scientist colleagues that what matters is less the relationship between old and new than the relative merits of successive theories through time.