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Conclusion

Why doubt?

My Descartes is not interested in refuting the sceptic; his focus is the scientific revoltion. Doubt is an alternative to the more direct approach taken in The World.

... to refute scepticism?

image: heisenberg, founder of quantum mechanics.
(a) you cannot refute scepticism,
and (b) scepticism is correct in this sense: the world is almost certainly quite different from how you think it is.

Doubt is necessary to establish ‘anything at all in the sciences that is stable and likely to last’

But refuting scepticism isn’t going to help establish anything in the sciences.
Why is doubt necessary? To lead us away from the senses!

‘although I feel heat when I go near a fire and feel pain when I go too near, there is no convincing argument for supposing that there is something in the fire which resembles the heat, any more than for supposing that there is something which resembles the pain.

There is simply reason to suppose that there is something in the fire, whatever it may eventually turn out to be, which produces in us the feelings of heat or pain’

\citep[p.~58, AT VII:83]{descartes:1985_csm2}

Meditation VI
Treatise on Man figure 1

Treatise on Man, figure 2

Why doubt?

Not to refute scepticism.

But because doubt is ‘the easiest route by which the mind may be led away from the senses’.

Note also that Descartes is quite dismissive of doubt by the end of the Meditations (earlier said could not distinguish dreaming from waking)

‘... the principal reason for doubt, namely my inability to distinguish between being asleep and being awake. For I now notice that there is a vast difference between the two, in that dreams are never linked by memory with all the other actions of life as waking experiences are.’

If, while I am awake, anyone were suddenly to appear to me and then disappear immediately, as happens in sleep, so that I could not see where he had come from or where he had gone to, it would not be unreasonable for me to judge that he was a ghost, or a vision created in my brain, rather than a real man. But when I distinctly see where things come from and where and when they come to me, and when I can connect my perceptions of them with the whole of the rest of my life without a break, then I am quite certain that when I encounter these things I am not asleep but awake. And I ought not to have even the slightest doubt of their reality if, after calling upon all the senses as well as my memory and my intellect in order to check them, I receive no conflicting reports from any of these sources. For from the fact that God is not a deceiver it follows that in cases like these I am completely free from error.’
\citep[p.~61--2, AT VII:89--90]{descartes:1985_csm2}

Meditation VI

... or?

‘I wanted to show the firmness of the truths which I propound later on, in the light of the fact that they cannot be shaken by these metaphysical doubts. ...

I could not have left them out, any more than a medical writer can leave out the description of a disease when he wants to explain how it can be cured.’

\citep[p.~121, AT VII:172]{descartes:1985_csm2}

Third Replies

conclusion

In conclusion, ...

Why doubt?

Even at the end,
the reasons Descartes gives for doubting
remain a mystery.

(Enlighten me!)