The Cogito argument

The statement which is necessarily true.
"I am"
or, "I am, I exist" is necessarily true

The name, cogito argument:
     cogito ergo sum. I think therefore I am

Suppose skepticism true, suppose that there is an evil demon.
Even so, this statement
     cogito ergo sum
is true whenever I state it or mentally consider it

What is the nature of this argument?

1. an inference.
     I think, therefore I am
the premise separable from the conclusion

     I think
     Whatever thinks exists
     Therefore I exist

2. This statement is necessarily true
     analytic (rather than synthetic) to use the jargon of philosophers

The statement "I exist" must be true, because its denial
     I do not exist
Is obviously false

Other analytic statements:
     Bachelors are unmarried males
     A triangle has three sides

The denial of these statements is obviously false
     Peter is a married bachelor
     This triangle has four sides

The cogito argument shows that I exist, but what am I?

Surprising that reasoning from such slim premises should yield substantial conclusions