What is a concrete example of logic in everyday life? This is not an easy question to answer, depending on the expectation of the response. If you want a concrete case in which you have to make a decision and expect logic to give you a clear, perhaps unexpected answer, I’m afraid you’ll be disappointed. Unfortunately, people often think of logic as a tool that is external, devoid of emotion, non-human. A tool that makes rational decisions, decisions that go against the emotional flow, against human nature.

When Spock decides to sacrifice himself to save the rest of the crew (and the universe?), he tells his captain: “Logic clearly dictates that the needs of the many outweigh the needs of the few.” This justification goes against the love and friendship his captain and friend feel for him, prompting his sacrifice. Once you read Logic in the Wild, I hope you’ll see that logic is not a discipline that dictates truths. Logic doesn’t adjudicate content; it guards coherence. It might be that Spock’s reason for his sacrifice is coherent with the rest of his beliefs, but that doesn’t mean logic prevents alternative choices.

If you expect a concrete example like Spock’s sacrifice to illustrate how logic becomes a guiding light when human emotion fails, but also an example more relatable than sacrificing yourself for the greater good, I’m afraid I’ll disappoint you. However, I can provide an example that isn’t so much guiding as it is protective. Logic is good at helping you slow down your thinking, to think it through. Humans have psychological biases that help us make quick decisions, which is indeed helpful in many situations, but in a complex society, it can be exploited.

Suppose I tell you that I’ll buy you a diamond ring if I love you, and I come back the following day with a diamond ring, will you think this confirms my love for you. Why? There are many other reasons I could buy you a diamond ring, such as tricking you into feeling secure in my love. I didn’t promise you that “if I buy you a diamond ring, then I love you,” but the other way around: “if I love you, then I’ll buy you a diamond ring.” Inferring from my promise that I love you when you get a diamond ring is committing the fallacy known as affirming the consequent.

A similar example, with the same logical structure, is this: There’s water on Mars if there’s life on Mars. NASA recently discovered water on Mars. Therefore, there’s life on Mars. Do you feel the same pull with this different content? The logic is the same, yet you might be fooled when it’s about love and diamond rings, but not when it’s about water and life on Mars. Just as with the diamond ring example, the fallacy lies in affirming the consequent. I didn't say that "if there's water on Mars, then there's life on Mars". I don't believe that. But I do believe that "If there's life on Mars, then there's water on Mars", because I believe that water is necessary for life. I wouldn’t infer from my belief and the discovery of water that there's life on Mars, nor should you infer that I love you from a diamond ring present under the promise that I'll buy you a diamond ring if I love you. That’s how logic can protect you—by guarding coherence in reasoning, allowing you to swap content while keeping the logical structure, and realizing when you’re being fooled by bad reasoning.

In many cases where Spock claims to follow the dictates of logic, it turns out he is fooled into thinking that his strong opinions and beliefs are the result of logical thinking when they aren’t. I’m afraid Spock is a bad role model when it comes to logic. Sorry geeks.