Kasser in the Systems Thinker's Toolbox defines them as such:

  1. The principle of plurality: plurality should not be posited without necessity, or don’t make things more complex than they need to be.
  2. The principle of parsimony: don’t do with more what can be done with less. This principle might also be considered as the basis for designing reliability into electronic systems, namely performing a function with the fewest components makes a more reliable implementation of the function; fewer parts to fail.

I don't see what's the difference between

don't make things more complex than they need to be


don’t do with more what can be done with less

  • The first one is about positing theoretically, the second one is about doing in practice. However, this distinction is specific to Kasser, and each of his items mixes the number of items with complexity, which are different things, and may potentially pull in opposite directions. These sorts of things commonly all go under umbrella labels like 'parsimony' or 'simplicity', concealing many different aspects, see SEP.
    – Conifold
    May 12 at 0:04

These two principles are both under the umbrella of Occam's razor. As Conifold pointed out, this distinction is specific to Kasser, not universally shared within philosophy realm. According to reference here:

While it has been claimed that Occam's razor is not found in any of William's writings, one can cite statements such as Numquam ponenda est pluralitas sine necessitate William of Ockham – Wikiquote ("Plurality must never be posited without necessity"), which occurs in his theological work on the Sentences of Peter Lombard (Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombardi; ed. Lugd., 1495, i, dist. 27, qu. 2, K).

Occam's razor, Ockham's razor, Ocham's razor (Latin: novacula Occami), or the principle of parsimony or law of parsimony (Latin: lex parsimoniae) is the problem-solving principle that "entities should not be multiplied without necessity".

Principle of parsimony are most likely used more often than principle of plurality as many philosophers prefer parsimony as their ontological commitment of a language such as Quine. It seems in Biology realm people also tend to use the term principle of parsimony as possibly influenced by the method of cladistic parsimony which is a type of phylogenetic inference that yields phylogenetic trees.

Cladistic parsimony (or maximum parsimony) is a method of phylogenetic inference that yields phylogenetic trees (more specifically, cladograms). Cladograms are branching, diagrams used to represent hypotheses of relative degree of relationship, based on synapomorphies. Cladistic parsimony is used to select as the preferred hypothesis of relationships the cladogram that requires the fewest implied character state transformations (or smallest weight, if characters are differentially weighted)... For a book-length treatment of cladistic parsimony, see Elliott Sober's Reconstructing the Past: Parsimony, Evolution, and Inference (1988).

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