Section Progress:

4.1 And Here Is Conjunction: &

The human brain is a pattern-recognition machine. It's helped us make amazing discoveries.

The philosopher Thales predicted the first eclipse in 585 BCE by recognizing patterns in the sky. Barry Marshall discovered that bacteria can cause ulcers by recognizing patters in his gut.

Sometimes our brains see patterns and intentions even where there aren't any, which is why the Greeks believed that floods and storms were caused by angering Poseidon.

BOOL: a logical system. The name is short for "Boolean logic".

In this chapter we'll use pattern recognition to our advantage. It's time to start building our first logical system.

A symbolic system deserves a symbolic name, so we'll call it BOOL.

We'll give you a few examples of how BOOL works, and you have to pick up on the patterns and figure out the rest. For example, here's how we say Pia is guilty and Quinn is guilty.

P&Q

Now you try.

As you figured out, capital letters stand for sentences, like P means Pia is guilty.

Pick the capital letter that makes the most sense, and follow the English as closely as you can.

Here's the basic rule of BOOL: pick the capital letter that makes the most sense, like you did for R.

Let's try another one.

Symbolic languages are unforgiving. Each sentence is a string of symbols with no spaces, and you have to get it exactly right, including capitalization.

It's like a programming language, where one wrong letter can create a syntax error that crashes the whole program.

But the strictness has an advantage: as we'll soon learn, it allows us to do computations with BOOL and prove that arguments are valid.

Don't let small differences in the English fool you, like the word "and" appearing between the names. When we translate into BOOL, we just want something equivalent.

When translating into symbols, we want something equivalent which mimics the English when it can.

Saying "Raquel and Pia are guilty" is equivalent to saying the longer "Raquel is guilty and Pia is guilty."

Our symbol "&" is only allowed to connect two sentences together. So we cannot say P&Q&R.

"&" can only connect two sentences.

The solution is to use parentheses for grouping.

Sometimes English doesn't group sentences clearly, but we still need to group them in BOOL.

If the English doesn't make clear, group from left to right.

Our rule is this: if the English doesn't make clear, then group from left to right.

The symbol & is called conjunction, or ampersand, or just and for short.

Conjunction, ampersand, and: &.

Conjunction is a type of symbol called a connective, because it connects onto other sentences to make new sentences.

Here's a helpful metaphor we will use a lot. Sentence letters like P, Q and R are called atomic sentences, or atoms, because they are the basic building blocks of BOOL.

If a sentence in English does not have another sentence as a part, like "Pia is guilty", then it is atomic.

Connectives combine with atomic sentences to make complex sentences. Complex sentences are the molecules.

As you've already seen, connectives can connect any sentences, atomic or complex, which is how we build up ever bigger molecules like this:

4.1 And here is conjunction: &