For most of us the words ‘logic’ and ‘common sense’ mean the same thing.

Both indicate a conclusion reached by moving sensibly, step-by-step towards an answer.

But that isn’t what either of these expressions really mean.

Very roughly speaking, formal logic is a process of observing the rules of deduction.

The answer itself is secondary: the means justifies the end.

Common sense, on the other hand, is more instinctive, more practical.

The answer is more important than the process: the end justifies the means.

Formal logic is a process that’s free from moral judgement.

With common sense, by contrast, the morality of the outcome takes precedence.

An example would be the way J S Mill modified Bentham’s definition of Utilitarianism.

Utilitarianism was how Bentham defined the purpose of civilisation.

Bentham’s logic made him define it as, “The greatest happiness for the greatest number”.

Mill, taking a more instinctive approach queried this: “What if hanging an innocent man makes a thousand people smile? Would that constitute the greatest happiness for the greatest number?

Mill’s point was that blindly following formal logic had not defined the purpose of a civilised society in the way Bentham clearly wanted.

So Mill took a common sense approach and modified Bentham’s definition: “The greatest good, ON BALANCE, for the greatest number”.

Mill knew that common sense allows ‘wiggle room’ whereas formal logic doesn’t.

Common sense, because it includes instinct and practicality, stops us behaving like machines.

Formal logic doesn’t judge the end result, whereas common sense very much does.

Because common sense allows for the unexpected, common sense allows us to remain in charge of the process by retaining awareness of the desired conclusion.

Bill Bernbach put it like this, “Principles endure, formulas do not”.

Logic adheres to formulas, common sense is flexible, it retains sight of the principle but is not rigid in the method of attaining it.

As J M Keynes said to a critic who accused him of changing his mind, “Of course when circumstances change I change my mind. Why, sir, what do you do?”

Which is why common sense is creative, whereas formal logic is not.

The strength, and weakness, of formal logic is its rigid discipline: “All birds have beaks. All tortoises have beaks. Therefor tortoises are birds.”

Of course, with common sense we know that all tortoises are not birds, so we know there is a fault in the logic, common sense allows us to start at the result and work backwards.

Which again, is why common sense is creative and formal logic is not.

This is a flaw in the education system.

What students learn at university is that good marks are given for the application of formal logic, and bad marks for simple common sense.

They learn that logic outranks common sense.

Which is why our marketing departments are full of people who rely on the application of logic no matter the evidence of common sense to the contrary.

The problem is that our audience, the people that receive our advertising, don’t operate on formal logic, they operate on common sense.

So we have people doing the advertising who are unable to work out why it isn’t working.

They follow the rules of logic they were taught at university, believing logic is superior to common sense.

The advertising may not work, but it doesn’t matter because logic isn’t governed by results.

But common sense tells us that logic isn’t what’s important.

Because common sense is only governed by results.