On babies.

My niece is nearly five months old, and every time I see her (usually every few weeks for a few days) her cognitive capacities are transformed from where they had been. Her body has changed, too: she grows, her legs thicken in preparation for eventual use, she makes new sounds, she sleeps through the night. (I.e. one six-hour stretch in the midst of one-to-two-hour chunks.) But even her bodily developments are in tandem with cognitive leaps and bounds. When my brother says "she's just discovered her legs this week," he means: her legs have come under her more direct and willed control. When she makes sounds that sound responsive, we know she isn't near speech yet, but also that this is how she becomes ready: she moves the muscles of her throat and jaw and tongue and sees what happens, and learns from it. That she has begun the years-long stage of grabbing at things means not that the muscles of her fingers and wrists have developed and not that her bones are stronger -- not mainly -- but that her brain has developed acces to new means of interacting with the outside world.

I won't say as much about the tiny teeth she can feel still perhaps months from breaking through the gum, but most of her development is in coordination and control. Her nerves are still learning to connect her muscles and vessels and organs and brain. She is in the process of what Descartes describes in Meditation Six, the pervasion of body by mind. No wonder that the older philosophers thought the soul had to be infused into the body, watching a small child; only that they believed it could be infused all at once.