# Are living organisms deterministic?

Are living organisms physically deterministic at any given time? Since it's all physics and chemistry, it leads me to believe they are.

• Since it's all physics and chemistry - i didn't get the reasoning. – user36790 Sep 19 '16 at 4:39
• This is not an answer because I'm no expert of the topic. I think since quantum-theory we know physics is not deterministic. But you don't have to go that far; when we predict how gases or liquids behave, we use "idealistic" models which are almost surely inaccurate on a certain level. So even if life is deterministic, the information required to predict it is not available and will never in the foreseeable future will be available. – mg30rg Sep 19 '16 at 8:43
• You might be interested in this: en.wikipedia.org/wiki/Laplace%27s_demon – Traubenfuchs Sep 19 '16 at 11:16
• Epigenetics shows why are not living organisms are deterministic. A little change during a cell multiplication and the whole organism is different. DNA allows for some divergence in parameters. youtube.com/watch?v=BD6h-wDj7bw – wigy Sep 19 '16 at 12:42

In the sense that most people mean, I'd say no because quantum mechanics isn't deterministic (at least the way most people think of "determinism").

Ultimately, to be more specific, I think this depends on your interpretation of quantum mechanics, and how reductionist you are.

The best you can do with reconciling quantum mechanics with determinism is taking a particular (admittedly relatively popular) interpretation of quantum mechanics (the many-worlds interpretation) which is "deterministic" but not in the sense you were probably thinking of, or in a sense which is very useful.

Reductionism, which is generally the working mode of thinking for scientists, implies that if quantum mechanics & physics is deterministic, then chemistry and biology are too because they are built only out of pieces of nature that are fully described by quantum mechanics and nothing else. If you want to be controversial, you can suppose that there exists "strongly emergent" phenomena which fundamentally can't be traced to quantum mechanics & physics (e.g. a soul which is not described by physics). This generally isn't very popular in experimental sciences because 1. it's not very useful and 2. these ideas generally don't have a great track record.

More on the quantum mechanics bit:
Quantum mechanics gives describes a system of particles and their evolution in time in terms of probabilities, not "deterministic" events, as classical mechanics would. And there's good reason (see Bell's inequalities and Bell tests) to believe that this probabilistic description is as fundamental as you can get for these things. The purest description of the universe is in terms of probabilities.

Probabilities seem manifestly non-deterministic. However, if you subscribe to the many-worlds theory then this is still in a sense deterministic, as quantum mechanics (or a similar theory) would correctly (and deterministically) describe the evolution of the state of the universe. Locally (i.e. in our part of the state which can't really observe other parts of the state) things look probabilistic, but if you could somehow observe the state of the whole universe, you'd deterministically predict how the whole thing evolves using quantum mechanics. If you don't know what I mean by this, I'd start by looking up the concepts of superposition and entanglement in quantum mechanics. As of yet I don't think there is anything testable about the differences between this and other interpretations of quantum mechanics (e.g. Copenhagen), so many people don't like reading into these things too much.

• I always thought, the uncertainty principle didn't actually state that the universe isn't deterministic, it just states, that there is no way to learn all the information required to predict the future, but the information is "out there". I mean, the best model to deal with the position of an electron is to assume it has a lot of simultaneous positions in a cloud, but it actually isn't a cloud. The cloud is an abstraction to handle uncertainty. I would rather say, the universe is deterministic, but the determinism is unpredictable. :D (I know-I know Occam's razor) – mg30rg Sep 19 '16 at 9:00
• I would think that when most people talk about determinism, they mean in the sense of a computer which, while fundamentally governed by quantum mechanics, is deterministic for most intents and purposes. If you stand by your idea of what "most people" mean by determinism, then it's worth pointing out that nothing at all is deterministic. That's not to take away from the fact that quantum indeterminacy is important to include in any answer, as you've done. – Jibb Smart Sep 19 '16 at 12:13
• We have no deterministic tools to learn the quantum state only random ones (indirect/statistical). We have deterministic tool to calculate dynamics: how a state turns into another state. Simple as that. We are not sure about the input, about the output, but we're pretty sure about the deterministic calculations which take input and produce output. These calculations are called quantum theory. – kubanczyk Sep 19 '16 at 12:39
• @mg30rg First i'll note that it's not specifically the uncertainty principle which makes quantum mechanics probabilistic, it the wave nature of the theory. It sounds like you are proposing what are usually known as "hidden variable theories" that would take away the uncertainty implied by quantum mechanics. These are attractive ideas, but they actually (surprisingly) make experimentally testable predictions that have have essentially been disproved (other than rather contorted versions) by Bell tests. That's what I meant when I mentioned "good reason" in my answer. – aquirdturtle Sep 19 '16 at 13:57
• I'm honestly not sure what you mean by "inferable hidden variables". The more specific phrase that I know is that Bell tests invalidate local hidden variable theories (i.e. you can posit non-local variables that don't belong to individual particles, but that gets weird I think). I'm also not sure what you really mean by "unpredictably deterministic". Sounds like you are talking about something like quantum chaos? – aquirdturtle Sep 19 '16 at 19:57

Note that systems are not deterministic, dynamical theories are. So you can look at a dynamical theory, and see if "deterministic" is an attribute of that theory or not, but all you can do with a system is ask "do I have a theory that predicts to satisfactory accuracy what that system does?" Here's the key point: if the answer to that is "yes", it still does not make that system deterministic, it merely makes the treatment that you are successfully using deterministic. At a higher level of precision, it is invariably true that your theory won't succeed any more, so what are you to say then? In the case of living systems, no deterministic theories have ever been successful in treating their behavior to any degree of accuracy. So the above gives us two independent reasons for rejecting the claim that living systems are deterministic: 1) we have no test if a system is deterministic, only if a theory is, and 2) no deterministic theories have ever worked well in all situations for any living systems. I suppose this allows anyone to believe what they like, but their evidence will be poor.

• "Note that systems are not deterministic, dynamical theories are" I don't think everyone would agree with that; that's very much entering the area of philosophy. – aquirdturtle Sep 19 '16 at 5:36
• They can agree or not, but my statement is clearly correct. Determinism is an attribute of a theory-- you can make of that what you will. How would you ever look at a system and know it is deterministic? But you can of course look at a theory and know it, because it's an attribute of a theory. – Ken G Sep 19 '16 at 5:38
• Suppose a system's behavior is perfectly described by some theory. If the theory perfectly describes the system, and the theory is deterministic, then I see no useful distinction to be made by still claiming that the system and it's behavior is not deterministic. – aquirdturtle Sep 19 '16 at 5:46
• How do you test your claim that a system's behavior is perfectly described? Is that something a scientist can tell? My point above was, every time a scientist says "theory A explains system B", they can never mean that system B is perfectly described by theory A, because that is not a testable claim. What they always mean is what the scientist can actually test: that theory A explains the observations to the precision that such observations can be made, in the regime in which they can be made. Consider the history of science. – Ken G Sep 19 '16 at 8:41
• Yes, the "perfect theory" is a limiting case in it passes every test scientists can conduct, but I think it's meaningful regardless. I'd also consider it meaningful to say that if a deterministic theory perfectly describes a system, as best as we can tell, then the system is deterministic, as best as we can tell. – aquirdturtle Sep 19 '16 at 13:50