Meditations by Marcus Aurelius Book 10 Page 34

place will fit thee. Unto the cylindrus, or roller, it is not granted to move everywhere according to its own proper motion, as neither unto the water, nor unto the fire, nor unto any other thing, that either is merely natural, or natural and sensitive; but not rational for many things there be that can hinder their operations.

But of the mind and understanding this is the proper privilege, that according to its own nature, and as it will itself, it can pass through every obstacle that it finds, and keep straight on forwards. Setting therefore before thine eyes this happiness and felicity of thy mind, whereby it is able to pass through all things, and is capable of all motions, whether as the fire, upwards; or as the stone downwards, or as the cylindrus through that which is sloping: content thyself with it, and seek not