The notion of Determinism is sometimes glossed as “every event has a cause”; but this characterization is problematic in a few ways (that I’ll explain below).
The way we’ll understand Determinism is instead this: it’s the idea that, given some past total “snapshot state” of our universe, there was only one possible way for things to develop from there. There was only one way the future could continue from that past snapshot.
Unpacking this a bit, Determinism is explained in terms of “laws of nature” that govern our universe. These are fundamental exceptionless generalizations about what the universe is like that are associated with some kind of “necessity.” Usually it’s assumed the laws of nature we’re talking about are the ones investigated by physics. But if you believe in souls, you can also include laws about how souls evolve and interact, and how they are connected to the physical world. (If there are any such laws. Some dualists think there are.)
Some laws of nature are such that, for given starting snapshots for the universe, those laws are compatible with only one subsequent future. Any universe in that past snapshot state but a different future would violate those laws. It couldn’t be a universe that those laws described or governed. We call laws of this sort Deterministic laws. Given a starting snapshot, these kinds of laws guarantee or require that the future will proceed a single way forward. Any two possible universes, if they started off in the same snapshot state, and both had the same Deterministic laws, must continue in the same way. Those laws make that continuation inevitable for those universes.
Other laws of nature are compatible with more than one subsequent future. We call these Indeterministic laws. These laws may say: given this starting snapshot state, the universe can evolve in any of ways A, B, or C. None of these possible futures is guaranteed to happen. At most, it may be that some of the options are more likely to happen than others.
We’ll be understanding Causal Determinism, or Determinism for short, to be the claim that our universe is at the most fundamental level governed by Deterministic laws.
Here are some presentations of this concept from texts we’ll be reading:
To say that a system is Deterministic means that everything that happens with it is the result of prior causes, and that once the causes occur the effects must inevitably follow, given the surrounding circumstances and the Laws of Nature. (Rachels, p. 483)
[D]eterminism is the view that at any time the universe has exactly one physically possible future. That is, something is Deterministic if it has exactly one physically possible outcome. (Lemos, p. 8)
Determinism is the thesis that it is true at every moment that the way things then are determines a unique future, that only one of the alternative futures that may exist relative to a given moment is a physically possible continuation of the state of things at that moment. Or, if you like, we may say that Determinism is the thesis that only one continuation of the state of things at a given moment is consistent with the laws of nature. (For it is the laws of nature that determine what is physically possible…) (van Inwagen, p. 268)
If Determinism is true, then, if the universe were “rolled back” to a previous state by a miracle (and there were no further miracles), the history of the world would repeat itself. If the universe were rolled back to a previous state thousands of times, exactly the same events would follow each of those thousands of “reversions.” (van Inwagen, p. 270)
How much evidence does science give us to believe that Determinism is true? Short answer: This is unclear. We’ll discuss it below.
First, some points of clarification:
Some philosophers have thought that without Determinism, there can be no causes and effects. But most disagree. Most think that there’s still causation even if Determinism is false. It’s just that causes will do things like changing the probability of what happens, rather than making what happens causally inevitable. This is one reason why we shouldn’t identify Determinism with the idea that “every event has a cause.” (Another reason is that the latter, but not the former, commits to the Big Bang having a cause.)
This is the reason why in earlier parts of our course, I preferred to talk about “overcausing” where other philosophers would usually say “causally overdetermining.” In those earlier discussions, whether the causing was Deterministic or not did not seem to be the central issue.
Some authors invoke ideas about what can be predicted when they explain Determinism. This introduces a number of complexities and controversies. Let’s not go there. The basic idea of Determinism is about what our laws of nature are like, and the past making the future inevitable. It’s not about what anyone can predict. Whether Determinism is true or false as a general claim about our universe might not even be something anyone’s in a position to know.
There is an important argument that many find makes a compelling case for Incompatibilism.
Some texts call this the Consequence Argument; others call it the Before You Were Born Argument.
The argument goes as follows:
If Determinism is true, then how we act today and in the future is the necessary consequence of the laws of nature and the way the world was before we were born. But it is not up to us what went on before we were born. And neither is it up to us what the laws of nature are. We have no control over those things. And if it’s not up to us whether certain things happen, then neither is it up to us whether the consequences of those things happen. If we have no control over the laws and the past, and they have the inevitable physical consequence that we will act a certain way, then we have no control over how we act. Hence, if Determinism is true, then it is not up to us how we act today or in the future.
This is called “the Consequence Argument,” because it appeals to the principle “If we have no control over certain things, then we don’t have control over the consequences of those things, either.”
Lemos presents the Consequence Argument on pp. 24–5.
van Inwagen presents the argument starting on the bottom of p. 273 and ending 2/3 of the way down p. 276. The claim I’ve highlighted in purple correspends to what he calls “the Principle.”
Where I've said that something is "not up to us," van Inwagen says that it's "untouchable," meaning that you are and always were unable to do anything to alter it (and couldn't have done anything even if you knew more or were more lucky).The Consequence Argument makes a very strong case for the conclusion that Determinism will be incompatible with free will. There are things a Compatibilist can say in response, but those things demand care and subtlety. Many philosophers do think these responses are good ones, or at least on the right track; but many others find them unsatisfying, and so they find the Consequence Argument convincing.
(As I mentioned earlier, in a 2020 poll, only 32% of philosophers favored some form of Incompatibilism; 56% favored Compatibilism; and 12% couldn’t choose.)
It’s sometimes claims that our experience of acting freely gives us evidence against Determinism.
That idea is countered with the objection: maybe it just feels as if we’re acting freely, when really we’re not. There’s a difference between feeling as though you’re in control, deliberating and making a choice, and genuinely having a choice about what happens.
Some findings from neuroscience are especially interesting here, as they suggest that our experience of acting freely may be distorted or fabricated in some ways. (To read more about these, see Act 4 in the Lemos text, and also these two podcasts, and these futher articles.)
Sympathy for the idea that our universe is Deterministic has a long history. Some of the ancient Greeks, especially the Stoics, thought that Determinism was true. The “clockwork” picture of the universe we got from Newton was widely understood to be a Deterministic one. But whether that understanding of Newton’s physics is correct has been challenged.
And nowadays, of course, our physics is even less straightforward than Newton’s. This makes the issue of Determinism even more controversial. It’s disputed whether General Relativity permits Indeterminism. And the question of Determinism become especially unclear when we turn to quantum physics.
Our best physical theories of quantum phenomena are not obviously Deterministic. They don’t say “in situations like this, so-and-so is guaranteed to happen.” Instead they involve probabilities in ways we can’t eliminate. However, it is philosophically controversial how the probabilities in these theories should be understood, and how other questions in interpreting the theories should be resolved. Hence, we cannot easily say whether or not our best theories of quantum physics entail that our universe has Indeterministic laws.
Probability is a kind of mathemtical structure. We can use that concept to describe various different things. By analogy, “degrees” are another kind of mathematical structure. But we can talk about degrees of rotation, degrees of temperature, degrees of trust, degrees of difficulty, degrees of influence, and more. These phenomena are pretty different from each other. Being able to talk about “degrees” in each case just means there is some mathematical commonality (there are also mathematical differences). So too with “probability.”
One place we talk about probabilities is to measure our evidence or limited information concerning an outcome. Suppose I roll a six-sided die, and it has already landed, but you haven’t yet seen which way it’s landed. Now I ask you, “What is the probability that the die came up one?” If you’re tempted to say, “1/6,” then you’re understanding probability to mean what it’s reasonable for you to believe. We call this an “epistemic” use of probability. This kind of probability changes as you acquire more evidence, even if the situation we’re talking about has undergone no change in itself.
Armed with that contrast, we can make a few points.
If our universe is Indeterministic, then some of the things that happen in it will either have to be outside the scope of any natural laws, or the laws that fundamentally apply to them will have to involve “objective” probabilities. In a Deterministic universe, the only “objective” probabilities you could have (at least at the fundamental level — it’s controversial after that) are 0 and 1.
It is philosophically controversial whether the probabilities appearing in our theories of quantum physics should be understood as epistemic probabilities or objective probabilities. If they’re epistemic probabilities, our universe may then be Deterministic after all. (But in ways that limit our ability to predict how it will evolve.) If they’re objective probabilities, our universe will be Indeterministic. We don’t know which of these pictures is right.
Even if the right way to understand quantum physics turns out to be as involving objective probabilities, and so as making our universe Indeterministic, still the kinds of indeterminacies it seems to introduce, according to our current theories, get “washed out” at the macroscopic scale. At the macroscopic scale, our laws of nature would still be so close to being deterministic that it arguably makes no practical difference. You’ll sometimes see theorists refer to our universe as “macrodeterministic.” What they mean is that at the macroscopic scale, it’s so close to being Deterministic that for all practical purposes it might as well be.
Finally, even if you think Determinism happens to be false about our universe, still you will learn a lot about our concept of free will by investigating what would follow if Determinism were true.
If you want to read more about probabilities and Determinism, and how to interpret the probabilities in quantum physics, you could start by reading these encyclopedia entries:
Here are some books that those who are experts on these issues say give excellent introductions and histories of the debate:
We said above that the concept of “probability” can be understood in different ways.
In ordinary folk talk, words like “chance,” “luck/unlucky,” “likely/unlikely,” “random,” and “chaotic” are used interchangeably with the concept of probability.
But in technical discussions in philosophy and other fields (math, physics, statistics, and so on), these words have specialized uses.
First, philosophers use the word chance to refer to probabilities, but only when they’re understood “objectively.”
Second, we can talk about luck whenever something significant is outside your control, and the result is unexpected or there’s a mismatch between it and your effort or preparation. So if you win the lottery but did nothing special to achieve that, that counts as lucky. (But if you win after somehow obtaining 90% of the tickets that were sold, that’s much less lucky.) In the other direction, if you exercise and eat healthy all your life, but nonetheless have a heart attack at age 40, that’s unlucky. If your friend has a heart attack after living and eating much less heathily, well that’s unfortunate for them, too, but we wouldn’t attribute it as much to bad luck.
The important point here is that “luck” in this sense can exist whether or not the universe is Deterministic.
The other concepts (likelihood, randomness, chaos) are used in specialized ways that we won’t explain here. (Sometimes they’re used in multiple ways. Some of their uses are also compatible with the universe being Deterministic.)