"Paddy and shamus" joke

Two builders (Paddy and Shamus) are seated either side of table in a rough pub when a well-dressed man enters, orders a Guinness and sits on a stool at the bar.
So Paddy and Shamus start to speculate about the occupation of the suit...
Paddy: I reckon he's an accountant. Shamus: No bleedin way, he's a stockbroker. Paddy: He ain't no bleedin stockbroker! A stockbroker wouldn't be seen dead in here!
The argument repeats itself for some time until the volume of Guinness gets the better of Paddy and he makes for the toilet.
On entering the toilet he sees that the suit is standing at a urinal. Curiosity and the several Guinness get the better of the Paddy...
Paddy: Scuse me sir... no offence meant, but me and me mate Shamus were wondering what you do for a living?
Suit: No offence taken! I'm a Logical Scientist by profession! Paddy: Oh! What's dat den? Suit: I'll try to explain by example... Do you have a goldfish at home? Paddy: 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? Paddy: It's in me pond! Suit: Well then, it's reasonable to suppose that you have a large garden then? Paddy: As it happens, yes I have got a huge big garden! Suit: Well then it's logical to assume that in this town, if you have a large garden then you have a large house? Paddy: As it so happens, I've got a five bedroom house... built it myself, I did! 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? Paddy: Yes I am married, I live with me wife and three fine kids. Suit: Well then it is logical to assume that you are sexually active with your wife on a regular basis? Paddy: Yep! Regular as clockwork four nights a week! Suit: Well then it is logical to suggest that you do not masturbate very often? Paddy: Me? No never ever. Suit: Well there you are! That's logical science at work! Paddy: How's dat den? Suit: Well from finding out that you have a goldfish, I've also found out all about your sex life! Paddy: I see! That's pretty impressive... thanks mate!
Both leave the toilet and Paddy returns to Shamus.
Shamus: I see dat suit was in there. Did you ask him what he does? Paddy: Yep, I did dat! He's a logical scientist! Shamus: What's dat den? Paddy: Look you here Shamus and I'll try and explain.
Do you have a goldfish?
Shamus: Nope.
Paddy: Well then, you're a wanker.