The "unsynthesised manifold" is, in the original sense, everything that is out there, regardless of whether we perceive it or not. As we can't sensibly talk about matters of which we are unaware, we can use the expression more usefully to describe the endless flood of undifferentiated sensory data we accumulate throughout our waking hours. Our conscious and subconscious attempts at organising this stuff and getting it to make a kind of sense are attempts at synthesis. Because of the way the brain routinely edits and translates the raw data, what we perceive is not reality itself but a model of reality as encoded by our individual software, even before we start trying consciously to make sense of it. Most of what we perceive evades conceptualisation, and is neither dreamed nor recollected, though sometimes we can fish it out under hypnosis.
If we are to comprehend the art object, we must turn off as much of the incoming noise as we can, and enter the silent space created by the work. It is as if, out of the roar of spam that is cyberspace, we were to receive a single one-word email. We have to agree, if only momentarily, to read it, even though the one word might be "nothing". Every time an artist constructs a word in neon and hangs it on the wall of a gallery, she or he invokes this moment of silence.