What form? What assumption?I think the problem is your assumption that an argument of a certain form cannot be objectively certain. You are going to have to make an argument for that.
Sorry, I'm not following you here. And Brain's claim was for "philosophical certainty", not "objective certainty".The argument is claimed to be objectively certain not because one does or does not hold to the conclusion as infallibly certain but because one should or has not right to deny the premises or the conclusion.