"People who do not agree with determinism are usually viewed with suspicion by rationalists who are afraid that if we accept indeterminism, we may be committed to accepting the doctrine of Free Will, and may thus become involved in theological arguments about the Soul and Divine Grace. I usually avoid talking about free will, because I am not clear enough about what it means, and I even suspect that our intuition of a free will may mislead us. Nevertheless, I think that determinism is a theory which is untenable on many grounds, and that we have no reason whatever to accept it. Indeed, I think that it is important for us to get rid of the determinist element in the rationalist tradition. It is not only untenable, but it creates endless trouble for us. It is, for this reason, important to realize that indeterminism - that is, the denial of determinism - does not necessarily involve us in any doctrine about our `will' or about `responsibility'. (Popper, Conjectures and Refutations, 1962, Harper, 1968, p.123)

"The idea that the only alternative to determinism is just sheer chance was taken over by Schlick, together with many of his views on the subject, from Hume, who asserted that 'the removal' of what he called 'physical necessity' must always result in

'the same thing with chance. As objects must either be conjoin'd or not, . . . 'tis impossible to admit of any medium betwixt chance and an absolute necessity'.

"I shall later argue against this important doctrine according to which the alternative to determinism is sheer chance. Yet I must admit that the doctrine seems to hold good for the quantum-theoretical models which have been designed to explain, or at least to illustrate, the possibility of human freedom. This seems to be the reason why these models are so very unsatisfactory.

"Compton himself designed such a model, though he did not particularly like it. It uses quantum indeterminacy, and the unpredictability of a quantum jump, as a model of a human decision of great moment. It consists of an amplifier which amplifies the effect of a single quantum jump in such a way that it may either cause an explosion or destroy the relay necessary for bringing the explosion about. In this way one single quantum jump may be equivalent to a major decision. But in my opinion the model has no similarity to any rational decision. It is, rather, a model of a kind of decision-making where people who cannot make up their minds say: `Let us toss a penny.' In fact, the whole apparatus for amplifying a quantum jump seems rather unnecessary: tossing a penny, and deciding on the result of the toss whether or not to pull a trigger, would do just as well. And there are of course computers,with built-in penny-tossing devices for producing random results, where such are needed.

"It may perhaps be said that some of our decisions are like penny-tosses: they are snap-decisions, taken without deliberation, since we often do not have enough time to deliberate. A driver or a pilot has sometimes to take a snap-decision like this; and if he is well trained, or jug-lucky, the result may be satisfactory; otherwise not.

"I admit that the quantum-jump model may be a model for such snap decisions; and I even admit that it is conceivable that something like the amplification of a quantum jump may actually happen in our brains if we make a snap-decision. But are snap-decisions really so very interesting? Are they characteristic of human behaviour - of rational human behaviour?

"I do not think so; and I do not think that we shall get much further with quantum jumps. They are just the kind of examples which seem to lend support to the thesis of Hume and Schlick that perfect chance is the only altenative to perfect determinism. What we need for understanding rational human behaviour and indeed, animal behaviour is something intermediate in character between perfect chance and perfect determinism - something intermediate between perfect clouds and perfect clocks.

"Hume's and Schlick's ontological thesis that there cannot exist anything intermediate between chance and determinism seems to me not only highly dogmatic (not to say doctrinaire) but clearly absurd; and it is understandable only on the assumption that they believed in a complete determinism in which chance has no status except as a symptom of our ignorance. (But even then it seems to me absurd, for there is, clearly, something like partial knowledge, or partial ignorance.)" (Objective Knowledge, Of Clouds and Clocks, 1972, p. 227ff)