The main implication of Descartes's statement is that it is difficult, if not outright impossible, to establish a meaningful connection between the thinking subject—the “cogito”—and the phenomenal world.

In Descartes's philosophy, a posture of radical doubt is necessary to illustrate this thought experiment. But once Descartes has established, to his own satisfaction, that the thinking subject exists, that still leaves open the problem of how the thinking subject can possibly relate to a world of objects and other minds.

In short, what Descartes's procedure of radical doubt gives us is a very shaky foundation on which to build a coherent philosophical system. If everything in the world other than the existence of the thinking subject can be subject to systematic doubt, then it becomes difficult to see how we can have any real confidence in the existence of objects and other minds.

At best, all we would be able to do is assume their existence for the sake of the system's overall coherence. But if that's the case, then most of the elements present in any system built on Descartes's foundations will lack the indubitable quality that he regards as essential for knowledge. In effect, we'd be left with a system which is mostly inexplicable.

When discussing Descartes's methodological approach to truth and skepticism, it is important to be aware of the sheer extent to which human knowledge was being transformed by the Scientific Revolution at the time. If you look towards the medieval era, intellectual circles were primarily centered around the great classical thinkers and philosophers, along with Christian tradition and scripture. From this perspective, the Scientific Revolution well deserves the title revolutionary: for the early modern era, knowledge could be built through examination and experimentation, and thinkers of this era radically challenged the assumptions and received wisdom of the ancients and the church.

Descartes's attitude and appliance of skepticism was very much shaped by this intellectual context. But despite his employment of radical doubt, Descartes was not a skeptic. On the contrary, Descartes's purpose was to formulate a new philosophical system that could withstand the tests of skepticism. His purpose in methodologically testing all of his assumptions about existence was to find a single insight or idea that was irrefutable, and from that foundation, attempt to prove the existence of the external world and the validity of his intellectual assumptions and endeavors.

For Descartes, the core insight on which the rest of his system is built is the statement: "I think, therefore I am." He arrived at this point after implementing a series of increasingly radical tests of skeptical doubt, a thought experiment that culminates with his supposition that, instead of there being a benevolent God, God might instead be actively malicious, aiming to mislead him at every turn. But even in this most extreme case, Descartes holds that he cannot doubt that he himself exists (at least in some form), given that there must be a thinking being running this thought experiment to begin with. Even if this malicious cosmic deceiver exists, there still must be a mind deceived. With this claim, he has found a statement about the universe that is unquestionable, and he can use this as his foundation in creating a stronger, more enduring, philosophical system.

In terms of the the philosophical implications of Descartes' statement, the idea becomes very clear that the ability to think is what defines consciousness.  The doubt inherent in the questioning of existence is resolved in that one can actually "think" and formulate the ability to doubt their own being in the world.  It is here where Descartes' statement holds profound implications because it takes nothing for granted, and seeks to construct a sense of the certainty within existence.  Descartes uses doubt as a way to construct certainty.  It is here where he is able to say that one can doubt all things and that which sustains such scrutiny can be considered real.  Descartes' use of doubt establishes the dualism between than which is living and nonliving, of subjects and objects.  It is here where I think that the Postmodern critique of Descartes can be linked back to his statement.  The ability to doubt everything is part of what makes his thinking so infallible in his own mind.  Yet, postmodern thinkers critique him because he fails to doubt his own dualism.  It is almost as if he uses doubt to merely substantiate a structure that he sees as impermeable to doubt.  Through this, one can use Descartes' own penchant for doubt in a larger context, one that questions everything and everything.  Cartestian logic through doubt can be applied to any and all conditions.  The ability to doubt the certainty of a dualistic mode of being would be a part of this.  It is here where I think that there is much in way of implications to Cartesian notions of doubt.