The third branch [of Science] may be called Semeiotike, or the doctrine of signs ... the business whereof is to consider the nature of signs the mind makes use of for the understanding of things, or conveying its knowledge to others.
[paraphrasing to make this shorter, he finishes with]
... since the things the mind contemplates are not visible it is necessary that something else, as a sign or representation of the thing it considers, should be present. And because the scene of ideas that make up one man's thoughts cannot be opened to the immediate view of another, to communicate our thoughts to one another (as well as record them), signs of our ideas are necessary.
Yesterday we talked about tightening bolts. When pondering what math subject I could address today, I thought about how we use signs and symbols as short-hand, to save on writing out full descriptions.
Here are a few less-commonly known symbols, used to save manufacturers the high cost of translating instructions into many different languages. They indicate which screws or bolts you can safely remove to take the back off a piece of equipment.
If you get stuck, try the Dictionary of Symbols by Carl Liungman