Whatever is real (if indeed there is anything real), it cannot be the same as what it merely appears to be. A table may appear to be of many different colors and shapes, depending on the angle from which it is viewed, but this does not mean that the real, physical table is undergoing any changes.
Descartes famously doubted all except, finally, cogito ergo sum. This is a good starting point for philosophy. Having doubted the existence of everything except for something-that-perceives (which need not be an enduring I), how can we allow ourselves to believe in external reality?
Russell says that believing in external objects is the simplest explanation for why we perceive the things we do, and there's no good reason to believe any other explanation, so we might as well accept it.
Matter seems to have some properties that seem to correspond with the sense-data caused by that matter, but we can't really know anything about any of this other than, Russell says, of the relations between things. Greater or lesser distance, but not absolute distance, for example.
Bishop Berkeley (George Berkeley, 1685–1753) suggested that objects exist only in perception, and the persistence of apparently real physical objects is because they are always perceived by God. Therefore, the correspondence of different peoples' perceptions of objects is not due to the external reality of the object, but due to the sharing in God's perception.
Distinguish acquaintance and description:
Russell addresses the problem of induction.
Russell describes a problem like Carroll's "What the Tortoise Said to Achilles" (Carroll, 1895).
Various logical principles are understood a priori, and cannot be proved by experience. Also, Russell asserts, "knowledge as to ethical value", meaning whether one thing is more desirable than another, is a priori, as is all pure mathematics.
Call as universals Plato's forms, and particulars the objects that share in those universals. Russell indicates that even if we wish to deny that universals such as 'whiteness' do not exist, as such (but are rather verbal conveniences to speak of similarities between objects of a given class, i.e. white things), we cannot deny that relationships (such as 'similarity') between two objects exist, and these relationships are themselves universals.
The confusion, Russell asserts, is that philosophers have focused on universals that are named by adjectives and nouns, ignoring those, such as relationships, that are typically named by verbs, prepositions, etc. So universals have been viewed as qualities predicated of a single object, and the discussion has been thus limited.
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