Why? Well, I... Look, I'm no four-dimensional expert on the subject...

6 seconds sound clip from the X Minus 1 - A Gun for Dinosaur by L. Sprague de Camp classic radio drama series.

You can hear this line at 00:08:59 in the radio play.

Quote context

[...]

- Well, to be exact, they have a little bump about the size of a tennis ball on the top of their spines, and you're not likely to hit it when it's imbedded in a six-foot skull.

- Try for the heart. They have big hearts, over a hundred pounds.

- An exploding shell in the heart will slow them down, at least.

- Oh. Oh, I see.

- Why do we have to go so far for game? Why couldn't we just go back fifty years and shoot lions in Africa?

- Well, the machine won't work more recently than a hundred thousand years ago.

- Why? Well, I... Look, I'm no four-dimensional expert on the subject...

- It has something to do with what they call time paradox. You know, if people could go back to recent times, they might do something to affect history or kill their own grandfather, you know.

- And there's also some kind of taboo about sending people back to the same time again.

- Eh, it's paradoxes... mustn't have 'em.

- What would happen?

- I'm not sure but the university isn't taking chances.

- They've got about a billion years to cover... they won't run out of eras.

[...]