Describing the nature of grossness and externality, the attributes of the external world, he says that grossness means the pervading of more portions of space than one, i.e. grossness means extension, and externality means being related to separate space, i.e. co-existence in space. Thus we see that extension and co-existence in space are the two fundamental qualities of the gross external world. Now an idea can never be said to possess them, for it cannot be said that an idea has extended into more spaces than one and yet co-existed separately in separate places. An idea cannot be said to exist with other ideas in space and to extend in many points of space at one and the same time. To avoid this it cannot be said that there may be plurality of ideas so that some may co-exist and others may extend in space. For co-existence and extension can never be asserted of our ideas, since +hey are very fine and subtle, and can be known only at the time of their individual operation, at which time, however, other ideas may be quite latent and unknown. Imagination has no power to negate their reality, for the sphere of imagination is quite distinct from the sphere of external reality, and it can never be applied to an external reality to negate it. Imagination is a mental function, and as such has no touch with the reality outside, which it can by no means negate. Further it cannot be said that, because grossness and externality can abide neither in the external world nor in our ideas, they are therefore false. For this falsity cannot be thought as separable from our ideas, for in that case our ideas would be as false as the false itself. The notion of externality and grossness pervades all our ideas, and if they are held to be false, no true thing can be known by our ideas and they therefore become equally false. Again, knowledge and the external world can never be said to be identical because they happen to be presented together. For the method of agreement cannot by itself prove identity. Knowledge and the knowable external world may be independently co-existing things like the notions of existence and non-existence. Both co-exist independently of one another. It is therefore clear enough, says Vācaspati, that the certainty arrived at by perception, which gives us a direct knowledge of things, can never be rejected on the strength of mere logical abstraction or hair-splitting discussion.