As we've seen, those who believe in psychological determinism try to show that there's a contradiction between the principle of causality – when applied to the inner world of the mind – and human freedom. Scientific determinism tries to show that a similar contradiction obtains when the principle of causality is applied to the external world, the world outside the mind. Applied to this world, the principle of causality would have us understand all things as composing an immense chain of causes and effects, in which every element is connected. Each term is an effect in relation to its cause and a cause in relation to its subsequent effect. Now, assume that man has the power to act freely. A defining characteristic of a free action is that it can change the world in some way. But this requires that the world is changeable, that it must be possible for us to disrupt these causal chains at will. And the principle of causality makes this impossible, for it denies that there's any contingency in the external world. So our actions can't be free.

This isn't to say that there's no such thing as freedom – only that, according to the scientific determinists, it remains bottled up in consciousness and can't express itself in the world outside the mind. So freedom has only a virtual value. We possess it, but we can't do anything with it.

But the scientific determinists actually go further.