"Moreover, force, gravity, and words of that kind, are often, and not unwisely, used in the concrete; in such a way that they know the movement of the body, the difficulty of resistance, etc. But when they are used by philosophers to signify certain natures, precise and abstract from all these, which are neither subject to the senses, nor can be understood by any power of the mind, nor can they be shaped by the imagination, they in turn give rise to errors and confusion."