Meta's errors include only thinking of something being either in the world or in the mind. So money, property and number, amongst other things, cause him great difficulty because they rely on communal intent. We might be tempted to express this as "they exist between minds", but that's not quite it, either. Some - many - things owe their existence to public rules, practices and recognition, and these need both minds (plural) and the world. Meta is trapped, as
notes, because if numbers are only in the world, he owes us a story about where they are; and if they are only in the mind, he owes us a story about how we manage to do things with them in the world.
Numbers are not like rocks, nor are they like sensations.
That's part of the reason that he can't make sense of logical precedence, restricting himself to temporal or spatial precedence. His metaphysical picture cannot represent logical priority at all, since it's neither purely mental or purely of the world. And along with that go other things that rely on public standards for correctness, such as normative dependence, and rule-dependence.
The following makes his error particularly clear:
The only way that "1" can refer to an object called "a number", instead of referring to distinct ideas in the minds of individual subjects is platonism. Platonism is the only way that "1" can refer to the same thing (a number, an object) for multiple people. Otherwise "1" refers, for you, to the idea you have in your head, for me, to the idea I have in my head, and so on. This is the way that values such as mathematical values are presumed to be objective rather than being subjective like many other values. It's known as platonism. — Metaphysician Undercover
Notice that this odd position is blandly asserted, not supported by any argument.
He relies on presuming that all reference must be object-reference, that object-reference must be either mental or Platonic, and that public sameness requires numerical identity of a referent. Meta relies on an unargued slide: “same object” → “same referent” → “same use” He treats these as equivalent, but they are not. What is required for reference to function is not that we talk about the same object but that we have a public criteria for correctness. It's learning that public criteria that
so clearly portrays; learning to count is learning to participate in public activities involving fingers and toy cars and slices of pizza. Numerals get their identity from roles in activities, not from reference to entities.