Gareth Evans (1985) developed several arguments for the Kantian thesis that our concept of things existing objectively necessitates conceiving of those things as existing in space. One of Evans’ arguments is what I call “The Causal Ground” argument. The gist of the argument is as follows. Our conception of objectively existing things involves attributing sensory properties to them. Sensory properties are dispositions to cause certain experiences in us. However, if sensory properties were pure experience-related dispositions, it would be unclear how we could conceive of objective existence, of perceptible objects existing unperceived. We must, instead, attribute, in addition to sensory properties of objects, an abiding causal ground of the dispositions in question. The conception of an abiding causal ground of sensory properties cannot be constructed merely of sensory properties themselves. We conceive of this causal ground in terms of primary, not secondary, qualities and thus as inhering in spatially extended material objects.
Evans’s argument concerns the spatial requirements on the objectivity of the objects of experience, on the things experienced. I want to address the question of whether a similar conclusion follows for the subjects of experience. Not only do I conceive of things that I experience yet exist independently of my experience, I also conceive of other subjects of experience that exist independently of any of my experiences. In short, I conceive of the objective existence of other minds. In part, this conception of other minds involves the conception of subjects of experience independent of me that are nonetheless able to have experiences of (at least some of) the same objects that I experience. Not only must the sensory properties of objects have abiding causal grounds, but the experiences themselves must have abiding causal grounds. In what inhere the abiding causal grounds of experiences themselves? They cannot be in the objects experienced, otherwise they would not suffice to account for occasions in which something is experienced by one subject but not another. Nor can the abiding causal ground of the experiences had by other minds be identical to my own, for this would fail to make these minds other minds. How, then, are we able to conceive of other minds? We do so by conceiving the abiding causal grounds of the experiences of distinct subjects as inhering in distinct spatially extended material objects.
One might ask whether the qualities of the experiences themselves suffice to distinguish the various subjects of experience. This might seem promising when we think of distinct subjects viewing a perceptibly asymmetrical object. However, it is possible to conceive of distinct subjects viewing a symmetrical object in such a way that the qualities of their experiences are identical.
Evans, G. (1985) “Things Without the Mind” In Collected Papers. New York : Oxford University Press.