From M43 the Mahavedalla Sutta:
It is said consciousness. Friend, what is consciousness?: It knows, therefore it is called consciousness. Knows what? Knows this is pleasant, this is unpleasant and knows this is neither unpleasant nor pleasant. Knows therefore it is said conscious. Friend, this knowledge and this consciousness, are they associated or dissociated? Is there a method to differentiate them and show them apart? What is known is consciousness and consciousness is knowledge. Therefore these things are associated and not dissociated and it is not possible to differentiate them and show them apart.
then later:
Friend, this feeling, perception, and this consciousness, are these associated or dissociated? Is it possible to differenciate them and show them apart?: Friend, feelings, perceptions and consciousness are associated and not dissociated and it is not possible to differentiate them and show them apart: Friend, the felt is perceived, and the perceived is consciously known Therefore these things are associated and not dissociated and it is not possible to differenciate them and show them apart.
If these three - consciousness, perception, and feeling - cannot be told apart, how are they conceptualized separately in the first place? It's a contradiction. If they cannot be told apart, why bother listing them as 3 separate aggregates? Why not just say the three arise together?