This is sometimes called the attempt to define god into existence, and was first proposed by Anselm of Canterbury (1033—1109). This original version was busted by Kant and Hume amongst others, but lo and behold, it resurfaced after several reworkings. While modern apologists are mightily proud of the shiny new gloss this has given the argument, it still devolves to the same thing:
A thing that can be imagined to exist, must exist, if it is imagined to have certain properties.
Clearly this is nonsense. However the dense fug of philosophical obscurantism is, as usual, used to hide the central argument, so let me expand what it says:
God is a being greater than which none can be conceived (unsubstantiated premise.)
Flying out to the Phils a while ago I watched an excellent show presented by the late Professor Stephen Hawking about time travel. This has fascinated us for over a century, leading to all sorts of speculation about time machines. HG Wells’ eponymous book is perhaps the first and defining of the many books about this and, of course, the beloved Doctor Who is dependent on the notion.
Time and Space
Wells’ vision, however entertaining, overlooked one major problem with time travel: even if you could move to a different when, establishing where that would be is staggeringly complex. The Earth is not stationary, indeed nothing in the Universe is. You would need to calculate precisely where in space you wanted to be at the time you wanted to be there. Suppose you just want to go back a week. It’s not just a matter of going back a week where you are now, because last week, where you are now was 11.25 million miles away.