Well, I think we are making heavy weather of Baby Logic. Arguments consist of premisses which allege to support conclusions. 'Support' traditionally amounts to 'provide reason to believe
the conclusion is true.' Reasonable belief is reasonable because the belief is thereby more likely to be true. Something is ultimately playing that role, or one can't make sense of support or
reasonable.  That's all I meant. Bryan wrote: 'I'm not really interested in whether or not something is "true". I am interested in whether or not a model works. I may say "true", but that's a short-hand for "This model manages to be a better fit to whatever data are available than the other models I've come across." Fine with me.

No, it's an argument all right. My very point. Non-deductive arguments are part of theory construction. I say to you: 'The planet wobbles at this point in its orbit. I infer that there is a large object affecting it, one we can't yet see. That's my theory.'  There is a premiss, a conclusion and an inference, the act of drawing the conclusion from the premiss, which in fact supports the conclusion--though it hardly proves it. I offer the hypothesis on the basis of the argument, I might say: 'Here is an argument that helps motivate my hypothesis that there is an unseen planet,' and then give the above argument.  This is not an overly expansive notion of 'argument,' it satisfies the definition, and, necessarily, every argument contains an inference. It just isn't a deductive argument (or inference), and to deny it's an argument on this ground, in a discussion about whether there are non-deductive arguments playing this role in science, is question begging. This is how the hypothetico-deductive model sometimes begins.