Descartes' second meditation seems to identify thinking in terms of all conscious acts when he describes a thing that thinks as "a thing that doubts, understands, affirms, denies, is willing, is unwilling and also imagines and has sensory perceptions-. Nevertheless, if Descartes' definition of thought was this broad it would need to be possible to assert that having sensory perceptions was as unquestionable as the ability to think. However, as Descartes stresses in the first meditation, it is not the case that believing, "I have sensory perceptions- automatically makes it a true belief. Regardless of this difficulty Descartes' inclusion of imagination in the above description of thinking demonstrates that he clearly does not understand thinking in the narrow sense "of being simply a cognitive act devoid of creative thought. When Descartes states that it is necessary to doubt, to understand, etc. he is essentially referring to the ability to be sentient of these functions rather than being able to control the activities themselves. Thus, the statement that "I am aware that I have sensory perceptions- is as indubitable as "I am aware that- and thus is likened with I think'. The conclusion can be reached that "thought- to Descartes can be defined as "that of which I am immediately aware- and at this point Descartes' method of universal doubt has left him disbelieving almost everything around him; for he would need to tear everything down in order to build anew. So Descartes has relinquished anything he could call into doubt and is rebuilding a method for certainty.
To go back to the subject of how Descartes can be certain that he has a true belief. If Descartes states, I think' then this is required to be proper because the act of thinking, the statement itself proves it. Of course, I think' is the only concept that can be self verified - thinking I exist' does not make it true.