But who am I ? A thing that thinks. What’s this ? One who inquires himself, understands, says, denies, wants, doesn’t want, always imagines and feels.
Therefore, from the simple fact that I know about my existence, besides the fact I am a being who thinks, I rightly conclude that my essence lies in the fact that I am only a being who thinks.
And yet… I have a body that I’m very tied to… And on the one hand I have a distinct and clear idea that is mine as the being that thinks; and on the other hand I have a distinct idea of my body… and I’m sure I’m different from my body and I can exist without it…
But best nature teaches me that I have a body that feels pain and that’s unpleasant to it, that it needs food or drink when I suffer from hunger or thirst; so I don’t have to doubt that there is something real in this matter.
Nature shows us through our senses of pain, hunger, thirst, etc., that I’m not only present, in relation to my body, as the sailor is present on his ship, but I’m tightly tied up to the fusion of my body that I make a single thing with it.
Otherwise, when the body is hurt I – who are only a being that thinks – would not feel pain, but I perceive that pain only through my intellect as the sailor realizes by seeing that something cracked on the ship; and when my body needs food and drink I would understand clearly, and my senses wouldn’t be troubled by hunger and thirst.
from R. Descartes, Meditationes de prima philosophia