Freedom can mean simply an absence of constraint. Even an inanimate object can be free in this sense. A rock dislodged from soil and rolling down a hill can be said to be free. A ship seized by arctic ice can be made free with the turn of the season. Freedom as absence of constraint has meaning also in the plant kingdom. A root-bound shrub can be released from a constraining pot and have the freedom of the near garden soil. Birds, of course, can be free — they are the very icons of freedom — as can others in all the phyla of the animal kingdom, including sapiens. Freedom, then, can do useful work as a purely descriptive term meaning absence of constraint. Perfect freedom and perfect constraint are imaginable, at least in the way Euclid’s points and lines are imaginable, but as a practical matter freedom and its opposite are matters of degree. A prison inmate generally isn’t free to leave the prison, but can enjoy degrees of freedom within the prison. A boat at a mooring isn’t free to float away, but isn’t entirely immobile either. A bird has the freedom of the air, but can fly only as high and as far as its body and the laws of physics permit.