You conclude that this proposition, I am, I exist, is true whenever it is put forward by you or conceived in your mind. But I do not see that you needed all this apparatus, when on other grounds you were certain, and it was true, that you existed. You could have made the same inference from any one of your other actions, since it is known by the natural light that whatever acts exists.

[Fifth Objections: CSM 11 180]

When you say that I'could have made the same inference from any one of my other actions’ you are far from the truth, since I am not wholly certain of any of my actions, with the sole exception of thought (in using the word ‘certain’ I am referring to metaphysical certainty, which is the sole issue at this point). I may not, for example, make the inference ‘I am walking, therefore I exist’, except in so far as the awareness of walking is a thought. The inference is certain only if applied to this awareness, and not to the movement of the body which sometimes – in the case of dreams – is not occurring at all, despite the fact that I seem to myself to be walking. Hence from the fact that I think I am walking I can very well infer the existence of a mind which has this thought, but not the existence of a body that walks. And the same applies in other cases.

[Fifth Replies: CSM 11 244]