On the personal level, retrograde planets are sometimes said to “internalise” the action of a planet, i.e., make it more yin, receptive, inward-looking, e.g., Mercury retrograde might listen more than speak. Perception is an internal process which seems appropriately represented by Mercury retrograde, and earthy Capricorn, a big fan of the tangible, seems an ideal place to touch and feel things. Mercury retrograde is a fine time for a short journey inward …
If I see an object in front of me, it means that certain neurons have fired in a particular way, and my brain has constructed an image based on that data. The process of seeing has happened entirely inside my own head, and consists solely of chemical reactions in my own nervous system. It seems unreasonable to assume that the resulting mirage in my mind should bear much relationship to “external reality.” The assumption that it does is sometimes known as Naive Realism. The same applies to all the other senses; sensory perceptions are only a factor of the perceptive apparatus that generates them; a tree that falls when no-one’s around doesn’t make a sound because sounds are generated by ears not trees.
By this logic, we can’t verify that anything we perceive exists outside of our own imagination. We can’t verify that it doesn’t either. In other words, we “know” pretty much nothing. Mercury, the Trickster, usually amply demonstrates this fact for us during his retrogrades.
Astrology is said to originate from Hermeticism, a non-dual philosophy where the mind (Mercury) is considered to be the cause of all imagined duality – hence its sign Gemini, the twins. Everything the mind can conceive has an opposite, and therefore each side of an opposition creates and necessitates the other; either both or neither can be, because essentially they’re the two ends of the same stick. This is why it isn’t possible to be e.g., happy all the time; happiness is a concept which simultaneous gives birth to its opposite concept, unhappiness. The two are twins, born at the same time, creators of each other; we can only take both or neither, because they’re one thing, and this applies to all mental concepts.
It’s said that “beyond the mind” (inwards) where Mercury by definition cannot go, is the peace which has no opposite, and this is often described as rock-like, and as the only thing that is in fact real (Capricorn).