I promised you last time that I’ll talk about this, so here it is. But I can’t talk about it in any of the normal ways available to me, as I’m supposedly weird (as I’m supposedly good at maths too) or so it goes. Let’s be mathematical about it. (I am weaving the story, remember, so I can pick whatever basically I like as the starting point. And I’m picking the **exclusive or** operation for some reason.)

**Exclusive or** is a logical operation that outputs true only when both inputs differ: it would be true if one is true and other false. If you have more than one input then the things begin to become more interesting, until you get to the point that you realise that XOR (**exclusive or**) is true only when an odd number of inputs is true… So in other words it’s reasonably easy.

Why am I going on about this? Well you will see that, plus a few other things in the following video.

Did you get that? (Oh by the way, do you know who Paul Erdős was?) Well what Sidon really said was mathematically quite correct. You have three operands here:

- you are turning up to see him (this would always be true as you are determined to see someone – a mathematician presumably)
- there is a person to see
- that person is in a place that you are coming to.

Now you have three inputs, and you would have to have all three true for this exclusivity principle to work out. So all of the above have to be true for this principle to work, and your visit to be successful. Ah, I’ve forgotten to tell you something. Exclusive disjunction could also be interpreted that ‘if and only if one is true, the other cannot be true’.

In the above case, you have a person who doesn’t want to be seen. Unfortunately for you. So you would have to make all the other inputs different – you wouldn’t turn up to see him, there would be no person to see, that person and a place you are coming to are not there.

Well to be a kind person as mathematicians are, and Sidon obviously was, he just excluded himself, rather than the whole story. So you see, he said that you could

- turn up to see a person
- there is a person to see
- that person is in a place that you are coming to

it’s just not him.

He wasn’t weird at all in my books.

Why I used XOR however may remain a mystery. If it is true, then the story (or title even) can’t be or something like that.