Demosthenes and questioning the gods

One of our regular library patrons always has interesting myth tidbits for me, since he knows I'm a myth geek. Today, he and I got talking about philosophy, and I started trying to track back a quote he was pretty sure belonged to Demosthenes. The gist was this: Man created the gods and myth to explain the world. So I had to do some looking, because I thought that was actually pretty early for a thought that more often gets attributed to Marx ("Man creates God, then gets on his knees and worships his invention.") or Rousseau ("God created man in his own image. And man, being a gentleman, returned the favor."*). Though Demosthenes was a contemporary of Aristotle, and was therefore post Socrates, I had assumed that the rationalist thought of that era tended to leave the gods where they were and get on with logical thought, rather than bothering about their existence.

The best quote I can find from Demosthenes online that goes along with the general idea is this: "Nothing is easier than self-deceit. For what each man wishes, that he also believes to be true." (This rather seems the principle around which quite a lot of urban fantasy novels, alongside the works of Douglas Adams and Terry Pratchett, are based, as it allows the supernatural to exist alongside the mundane without the mundane world giving much notice.)

So I'm putting the call out to all of the intelligent and wonderful (and possibly better read in ancient philosophy than me) people who read this blog: is there a Demosthenes quote like the one my acquaintance recalled? Is the concept of inventing mythology as explanations as old as the Greek rationalists, or is it more of a modernist notion?

--

*I had actually always attributed this quote to Mark Twain, though substituting "gentleman" for "likable sort." I've always liked it, because I felt it commented more on human consciousness than higher powers of any sort, but that could just be my reading of it, out of context.