I am someone who believes that only the actual is possible, that everything that is, is necessarily the case. I believe that position is a strong form of actualism, often called necessitarianism. I know it is not a very popular position among philosophers. But I wonder, what are some texts and books and papers that defend necessitarianism? I ask because I think necessitarianism has some unwelcome or at least unintuitive implications in my view. Some such implications are: that every true statement about the physical world is a law of physics, and that modal scientific definitions are vacuous. I would be especially interested in some texts that both advocate necessitarianism while at the same time preserve our intuitions.


2 Answers 2


Necessitarianism has more than one sense. In a general sense it means that everything that is true, or everything that happens, could not be otherwise. In the philosophies of the continental rationalists, this is related to the principle of sufficient reason. It is also understood to be in opposition to libertarian theories of free will. The classic defenders of this position are Spinoza and Hobbes.

  • Hobbes, Of Liberty and Necessity, 1654.
  • Spinoza, Ethics, 1677.

Some philosophers defend a version of necessitarianism under which all the laws of nature are necessary and hence hold in all possible worlds.

  • Brian Ellis, Scientific Essentialism, Cambridge, 2001.
  • Christopher Swoyer, “The Nature of Natural Laws”, Australasian Journal of Philosophy, 60: 203–223, 1982.
  • Sydney Shoemaker, “Causal and Metaphysical Necessity”, Pacific Philosophical Quarterly, 79: 59–77, 1998.

In more recent metaphysics, necessitarianism has been applied to the doctrine that everything that exists does so necessarily. Timothy Williamson is an important defender of this position, which he describes using the phrase, "Necessarily everything is necessarily something".

  • Timothy Williamson, Modal Logic as Metaphysics, Oxford, 2013. For a shorter exposition of Williamson's position, there is a paper called "Necessary Existents" from 2002 on Research Gate.

Some other literature on this subject:

  • A.N. Prior, "Past, Present and Future" (Oxford: Clarendon Press, 1967), 149-151.
  • Alvin Plantinga, "On Existentialism", Philosophical Studies 44 (1983), 9-10.
  • Kit Fine "Plantinga on the Reduction of Possibilist Discourse", in Alvin Plantinga, J. E. Tomberlin and P. van Inwagen (eds) (Dordrecht: D. Reidel, 1985), 160-180.
  • Timothy Williamson, "Bare Possibilia", Erkenntnis 48 (1998), 257-273.
  • Timothy Williamson, "The Necessary Framework of Objects", Topoi 19 (2000), 201-208.
  • Christopher Peacocke, "Principles for Possibilia" Nous, Vol. 36, No. 3 (2002), 486-508.

There are two ideas, developed by two very different thinkers, which I don't think call themselves necessitarianism, but which I think I could make a coherent argument are necessitarian philosophies.

Max Tegmark's Mathematical Universe

Stephen Wolfram's Ruliad

I would argue these ideas are isomorphic to each other - synonymous but defined from different angles - so which you should focus on just depends on if you prefer to conceive of the universe as Mathematical or Computational. Go for the Ruliad if you prefer a computational model, and Mathematical universes, of course, if you prefer Mathematical.

These ideas both amount to something like the following: ANY universe which is mathematically definable, or algorithmically definible, DOES EXIST. Thus, everything that is "possible" as strictly defined by those criteria must exist - somewhere in the multiverse. And YOU exist in this universe, with me, because this universe is among the universes that are definable, and thus we exist because we must exist, as beings in a definable universe.

These ideas may not be exactly what you had in mind when you said 'necessitarianism', but I think they should be considered as qualifying philosophies. And I think they're compelling as well.

When it comes to ideas of physics in this universe that can be described as necessitarian, there are some interpretations of Quantum Physics that fit the bill, like Many Worlds and Pilot Wave Theory.

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