Rene Descartes is considered the father of Modern philosophy. In an age of reason, Descartes wrote, "Cogito ergo sum" - I think, therefore I am. This defines existence as the ability to think rationally and to reason. See his Discourse on the Method.

Today, however, it is widely regarded that the Modern era has passed, and today we deal more immediately with ideas of postmodernism and posthumanism. In a postmodern/posthuman world, then, authority has shifted from reason to capital. Our existence, then, is now defined by our participation in the exchange of capital. Perhaps it is more informed to say, "I buy, therefore I am."