Quotes

A handful of good life is better than a bushel of learning. 
— George Herbert 

Descartes does not use this first certainty, the cogito, as a foundation upon which to build further knowledge; rather, it is the firm ground upon which he can stand as he works to restore his beliefs. As he puts it:

Archimedes used to demand just one firm and immovable point in order to shift the entire earth; so I too can hope for great things if I manage to find just one thing, however slight, that is certain and unshakable. (AT VII 24; CSM II 16)

.... the goal of Descartes in establishing this first truth is to demonstrate the capacity of his criterion — the immediate clarity and distinctiveness of self-evident propositions — to establish true and justified propositions despite having adopted a method of generalized doubt.

As a consequence of this demonstration, Descartes considers science and mathematics to be justified to the extent that their proposals are established on a similarly immediate clarity, distinctiveness, and self-evidence that presents itself to the mind. 

The originality of Descartes' thinking, therefore, is not so much in expressing the cogito — a feat accomplished by other predecessors, as we shall see — but on using the cogito as demonstrating the most fundamental epistemological principle, that science and mathematics are justified by relying on clarity, distinctiveness, and self-evidence. 

Baruch Spinoza in "Principia philosophiae cartesianae" at its Prolegomenon identified "cogito ergo sum" the "ego sum cogitans" (I am a thinking being) as the thinking substance with his ontological interpretation. It can also be considered that  

Cogito ergo sum is needed before any living being can go further in life".