Dubium sapientiae initium (Doubt is the origin of wisdom). - René Descartes

One needs to know what thought is, what existence is and what certainty is.

Pues tales frutos he recogido ya de ese método, que, aun cuando, en el juicio que sobre mí mismo hago, procuro siempre inclinarme del lado de la desconfianza mejor que del de la presunción, y aunque, al mirar con ánimo filosófico las distintas acciones y empresas de los hombres, no hallo casi ninguna que no me parezca vana e inútil

By 'God', I understand, a substance which is infinite, independent, supremely intelligent, supremely powerful, and which created both myself and everything else […] that exists. All these attributes are such that, the more carefully I concentrate on them, the less possible it seems that they could have originated from me alone. So, from what has been said it must be concluded that God necessarily exists