Copied from an Australian web site:

Two builders (Chris and James) are seated either side of a table in a rough pub when a well-dressed man enters, orders a beer and sits on a stool at the bar.
The two builders start to speculate about the occupation of the suit.
Chris: I reckon he's an accountant.
James: No way - he's a stockbroker.
Chris: He ain't no stockbroker! A stockbroker wouldn't come in here!
The argument repeats itself for some time until the volume of beer gets the better of Chris and he makes for the toilet. On entering the toilet, he sees that the suit is standing at a urinal. Curiosity and the several beers get the better of the builder...
Chris: Excuse me.... No offence meant, but me and me mate were wondering what you do for a living?
Suit: No offence taken! I'm a Logical Scientist by profession!
Chris: Oh! What's that then?
Suit: I'll try to explain by example... Do you have a goldfish at home?
Chris: Er...mmm... well yeah, I do as it happens!
Suit: Well, it's logical to follow that you keep it in a bowl or in a pond. Which is it?
Chris: It's in a pond!
Suit: Well then, it's reasonable to suppose that you have a large garden then?
Chris: As it happens, yes I have got a big garden!
Suit: Well then, it's logical to assume that in this town that if you have a large garden that you have a large house?
Chris: As it happens I've got a five-bedroom house... built it myself!
Suit: Well given that you've built a five bedroom house it is logical to assume that you haven't built it just for yourself and that you are quite probably married?
Chris: Yes I am married. I live with my wife and three children!
Suit: Well then, it is logical to assume that you are sexually active with your wife on a regular basis?
Chris: Yep! Four nights a week!
Suit: Well then, it is logical to suggest that you do not masturbate very often?
Chris: Me? Never
Suit: Well, there you are! That's logical science at work!
Chris: How's that then?
Suit: Well from finding out that you had a goldfish, I've told you about the size of garden you have, the size of house, your family and your sex life!
Chris: I see! That's pretty impressive... thanks mate!
Both leave the toilet and Chris returns to his mate.
James: I see the suit was in there. Did you ask him what he does?
Chris: Yep! He's a logical scientist!
James: What's that then?
Chris: I'll try and explain. Do you have a goldfish?
James: Nope
Chris: Well then, you're a wanker.