Thus She Spoke; Therefore I Listened

Be Word Wise

'Thus' Or 'Therefore'?

There is endless confusion about these two words.

Warning: the confusion is made worse by faulty (read, 'plain wrong') advice offered on several so-called 'language learning' sites.  

The difference between the two words is really quite straightforward.

‘Thus’ means ‘in this way’ (it’s the 'how')

If you want to say ‘in this way’, then use 'thus'. Chances are you won’t need to use it often. It can sound a little stilted. If it’s used properly, it can be replaced with ‘that’s how’. (So why not say ‘that’s how’?)

Example:
I found the combination, manipulated the dial and pulled open the heavy door of the safe; thus (that’s how) I retrieved my treasures.

People often write ‘thus’ when they should write ‘therefore’. The two are sometimes interchangeable but not usually. I’ll explain.

‘Therefore’ means ‘for this reason' (it’s the 'why')

‘Therefore’ is often about cause and effect and, in informal language, it can sometimes be replaced by 'so'.

Example:
It was raining; therefore (for this reason / that’s why), I took my umbrella.

Note that 'therefore' starts a new clause, and is preceded by a semi-colon, not a comma. 

In the example, above, ‘thus’ would be totally wrong. It’s not about ‘how’ you took your umbrella, but ‘why’.

When they seem to be the same…

Finally, look at these two sentences:

I saw he had a runny nose, a cough and a high fever; thus (that’s how) I decided he had flu.

I saw he had a runny nose, a cough and a high fever; therefore (for this reason), I decided he had flu.

They give you a good example of how ‘thus’ and ‘therefore’ seem to be interchangeable – not because they mean exactly the same thing, but because the meaning, or outcome, of the sentence remains the same. 

The simplest advice is, as always, to be absolutely sure about what you mean to say.