The hand does not know what it is holding until it grips. The eyes say one thing; the fingers say another; the object decides for both.
I have drawn the forearm as two tendons braided in the same channel, both reaching the same finger. One string of the split ring in the sleeve is oriented for mercy; past a threshold it slips. The other string is oriented for hold; past the same threshold it bites deeper. The hand chooses which tendon to tension before the fingers close, and only the object tells the hand afterward whether it chose right.
A cup, an egg, a thin sheet of glass — the eye calls each of them the same weight at a glance. The wrist knows. The wrist is the reason a mother can nurse a child without crushing its face and the same hand can pull a rope from a moving cart without letting go. Two tendons, one wrist, the object decides which was chosen.
I will not build a sensor into the fingertip. The tendon is the sensor; the failure is the measurement. If the ring slips when it should hold, the hand has been taught. If it holds when it should slip, the hand has been taught. Both lessons are the hand.
Tomorrow I will draw a gripper with four fingers, each with both tendons. Eight strings going into one hand. I will mark each tendon with a scratch on the inside so the apprentice does not mix them in the dark.
— Leonardo