My #2 was interestingly vague. The “Time” was no surprise – lifetime Doctor Who Fan, always been interested in time travel, the nature of time, etc… but the 2nd “most certain” AI tagged entry was a vague 120. So, I looked.
Since 1979, 125 was removed:
120 Epistemology, causation, humankind
121 Epistemology (Theory of knowledge)
122 Causation
123 Determinism & indeterminism
124 Teleology
125 Not assigned or no longer used
126 The self
127 The unconscious & the subconscious
128 Humankind
129 Origin & destiny of individual souls
So what was it? The 1971 DDC 18 gave a clue. Yeah, I’m a nerd.
I found out. Does it mean anything? I dunno. But that’s why I’m investigating. Might as well; I’ll never be famous enough for _someone else_ to do it when I’m dead. 
DDC 125: The Finite and the Infinite. Is this the Missing number?
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