Halliday & Matthiessen (1999: 453-4):
According to the first approach, verbs are really adjectives (their true shape is revealed in the adjectival participle), while the second approach assumes that adjectives are really verbs; they just happen to be superficially defective (in English) in that they cannot be inflected. The third approach aligns adjectives with ‘substantives’, but it still allows for two possibilities. They can be either treated as independent classes or they can be grouped together as nouns.