Cogito ergo sum (AT 25, 140-141, 422)
A. The syllogistic interpretation (see AT 140-141)
1. All thinking things exist
2. I think
3. Therefore, I exist.
B. The intuitive interpretation (see AT 422)
I immediately intuit my own existence whenever I think.
C. The predication interpretation
1. If a property can be truly predicated of x, then x exists.
2. A property (namely, thinking) can be truly predicated of me.
3. Therefore, I exist.
D. The substance-mode interpretation (cf. AT 44)
1. An existing mode must inhere in an existing substance.
2. My thinking is an existing mode.
3. Therefore, I (a thinking substance) exist.
Purposes of the cogito:
1. To give me a truth of which I can be unquestionably certain (AT 25).
2. To focus attention on my pure thinking as a source of certainty (AT 28-29).
3. To give me a standard of certainty: clear and distinct perception (AT 35-36).