With use of a Venn Diagram, this picture serves to illustrate what is formally referred to as an ‘intersection of sets’. Intersecting two or more sets means you’re isolating the elements that can be seen as mutual in all of them; elements that they all share.
Using Set Builder Notation, if we were to denote Science and Art by S and T respectfully, then the intersection of set S and set T would be written as S ∩ T. This can be read as “S intersect T”, or “S and T” and even “S cap T” for simplicity. Notice that this operation is a way to go about collecting all of the elements that are shared in both S and T.
How about if you were to want to look at the elements that are in both S and T, and S or T, all at the same time? Or, more explicitly, what if we were to want to see the elements in S, the elements in T, and the elements in S and T all at the same time?
This is where the notion of a “union of sets’ comes in, denoted S ∪ T. This notation is read as “S union T”, “S or T”, or even “S cup T”. Notice though, the use of “or” in this context; here we’re using what is called the ‘ inclusive “or” ‘, which is an extension of the truth-functional logical operator for inclusive disjunction.
In everyday life, there are two ways we can go about using the word “or” in discussion. Allow me to illustrate; If you were to ask me whether I want to go to Cuba or Japan next weekend, it’s intuitively obvious that I can choose one or the other, while choosing both is implicitly out of the question. However, if I were to ask you whether you would like pie or ice cream for dessert, I shouldn’t be surprised if you reply telling me that you’d like a little bit of both. In day-to-day conversation, our use of the disjunctive operator is highly dependent the content of the discussion.
Do you see how this could present a level of ambiguity in mathematics, where often context can be easily lost or difficult to translate due to abstraction? This is why we define our use of “or” in set theory (and the majority of mathematics and mathematical logic for that matter) as being that of the inclusive disjunctive operator.
Can you think of any other ‘operations’ that would be useful to define in a system such as the one we’re using?
What is it that makes something an ‘operation’ per se?
I look forward to hearing your thoughts!
