1.) Let the reasonable, whether intellectual or moral, be defined relative to an ideal knower or reasonable person. Thesis: this ideal knower must actually exist.
2.) Objections: (a) Ideal knowers are essentially hypothetical. (b) Ideal knowers are principles of knowledge and not principles of existence. (c) Ideal knowers are counterfactual and so their real existence need not even be possible.
3.) Respond: Ideal constructions in the physical world differ from those in the cognitive world. Physical ideals are either (i) facts about the world that make no difference in our measurements: assuming the earth is flat when making a mid-range artillery shot; assuming a test particle has no mass. This is not purely arbitrary or subjective: Measurement requires parameters, which allows (requires?) us to ignore what falls outside them. (ii) An attempt to isolate a principle which may never (or even can never) really exist in isolation. A perfectly smooth road illustrates purely inertial motion, even if no such motion could ever exist. These cases generalize: in the first something that really exists is unnecessary to our knowledge of it; in the second our knowledge of something does not require that it really exist.
But things that exist in the cognitive realm cannot always be subsumed under this general division, since their being is their being known. For example, error and truth exist in minds or not at all, and so to relate these to an ideal knower requires such a knower to really exist in order for these things to really exist within knowers that are not ideal. But by hypothesis a speculative error is that which is erroneous to a knower with all possible information, and so such a knower actually exists.
4.) All objections relate to idealizations of extra-mental reality, as mentioned in the response.