Analogue systems can't represent general concepts
Analogue devices only capture particular sensory patterns. They cannot (by themselves) be used to recognize and process unversal concepts.
For example, the retinal image of a chair is insufficient to represent the universal concept of a chair. The retinal image must be recognized as a typical chair pattern and must be associated with a verbal label by some sort of digital mechanism.
Zenon Pylyshyn (1974).
Note: Pylyshyn allows that some analogue computations may be important in practice, and he thinks it is likely that practical AI systems will use hybrid analogue-digital mechanisms. He is arguing against the claim that all mental computation is analogue.