OK, thanks for clarifying.

I agree that if I don't have a crisp understanding of what it means for X to exist (such that maybe X implemented as a neuronal pattern exists, and maybe it doesn't, and no amount of data about the world could tell me which it is because I don't know how states of the world map to the existence or nonexistence of X in the first place ) then I can't clearly assert whether X exists or not.

For my own part, I'm fairly comfortable refusing to use "exists" to refer to that which abstract objects are doing by virtue of being represented by a particular neuronal pattern, and I'm consequently fairly comfortable identifying as a nominalist (for purposes of this question). If I were instead comfortable using "exists" to refer to that act, I would identify as a Platonist (fpotq). If I was comfortable doing both, or neither, I would choose "Other."