It was evening, and as I sat watching the darkness spread across the land I chanced to think about how strange a thing it was to see the light vanish all on its own. When I said as much to Rachel, my imaginary friend, I regret she failed to see why this puzzled me.

As if that wasn’t enough, I was confidently assured that “It’s because the sun’s gone down behind the horizon” – and with this and much other babbling I was instructed in the ways and habits of a rather peculiar solar entity that (being composed of a large burning ball) shines light on the earth, each side being illuminated in turn as the earth spins. I must admit I’m not sure I’d ever heard such nonsense in my life.

I told Rachel that “Whilst the guy who invented this sun theory you’re telling me about was obviously smart, yet it cannot be any more than an unsupported theory. There is no evidence for the sun. There’s no way we can know this supposed sun exists!”

The indignant reply came almost as if she had been studying ways to defend the theory. Rather smugly, she asked me if I had seen a large circle of bright light, rising behind one horizon in the morning, and setting behind the other in the evening – bringing with it light and warmth.

Having admitted as much, I asked the obvious question: “Why should my having seen this circular light have anything to do with the theory of the sun?” This threw her off balance somewhat – for some reason she thought she’d made the case rather well. After some thought she replied with slow and measured tones to the effect that this very circular ball of light is the sun, and so the fact that I have seen the light coming from it, felt the warmth of it, and witnessed its path through the heavens is evidence that it exists, just as she had said.

Said I – “Nonsense. Your supposed evidence relies upon your presupposition that the sun does exist. If the sun doesn’t exist, then the light from the sky was not from the sun! How can you expect me to accept evidence which is contingent upon something I deny?”

She brought up various other things as the night went on. Vain theories such as the supposed evidence of photosynthesis, gravitational studies, orbital calculations, optical measurements and the like. I listened graciously, gently shaking my head and grimacing as one invalid argument after another was brought forward.

Eventually I told her the bottom line. “It comes down to this”, I said, “If I am to believe in the existence of the sun, then it must be from evidence that does not arise from or as a consequence of the supposed sun. If you believe the sun exists only because of the revelation of the sun, then I’m afraid your arguments all fall flat, for if the sun does not exist then not a single one of your arguments is valid.”

To bring the point home I said in a persuasive and somewhat louder voice that “I will only be convinced of the existence of the sun if its existence is revealed to me by some other means than the revelation of the sun”

That did the trick. She rolled her eyes, shook her head, and walked off – clearly defeated and going to tell her physics professor that what he’d said about the sun was baseless and groundless.

How foolish and ignorant she is, believing in something based only on evidence of its own existence!