If I affirm an object, it exists.
if i affirm myself, i exist.
what i am doing here is: affirming an object as real, as being; affirming a subject as real, as being.
if i can make the further judgment that the object is not the subject, i have objectivity.
of course, i could specify further: in what way is the object not the subject? there is the objectivity of "2 + 2 = 4", of mathematical entities; there is the objectivity of a thought, or a dream; there is the objectivity of a snake that enters a goa house... of a new planet that someone claims to have discovered...
Descartes assumed that he knew the self in an apodictic way. he assumed also that he did not know any object in an apodictic way. hence his problem, the problem which we call the bridge.
he did not bother to engage in a detailed study of what was happening when he was knowing.
not that there is no difference between affirming the self and affirming an object that is not the self. there is a difference. but - and here is where each one has to get clear for himself - it is possible to know objects, and we do it a thousand times.
here you could in an idle moment read my notes on Heidegger (hermeneutics). Here you could reflect on Wittgenstein's unique handling of the problem.
one of Descartes' problems was his mathematical training: he expected, wrongly, mathematical certitude for everything. Even Rorty says that: Descartes was like the undisciplined child who does not know when to stop asking questions. (And Rorty, surprisingly, quotes Gilson quoting Descartes!) (I have this in my Rorty article.)
do we have mathematical certitude w.r.t. the reality of the self? not exactly. but to deny the self is to engage in a performative contradiction. (you can deny the self. the conradicition is not logical. it is performative: beween saying and performance, or reality, if you wish.)
do we have mathematical certitude w.r.t. objects like a cat and a mat and a tree and a dog and ohther subjects? no. do we need such certitude? no. do we have the self kind of certitude in these cases? no. but we do come to the point where to ask furhter questions would be plain foolish (even Rorty!). "the spade hits rock" Wittgenstein says. Absence of further relevant questions, L says. physical certitude, perhaps the Thomists would say...
If you need some references:
Hermeneutics and Method, p. 26 para 2.
Verbum cwl 2, p. 98 last para.
Insight, cwl 3, p. 401 last para. the pseudo-problem of transcendence.