What is the difference between therefore and thereby?

Therefore – as a consequence of something, as a result.

Thereby – by means of something, in that connection or relation.

Both are adverbs and they are essentially stating the same thing with both of them having eerily similar definitions.

I disagree that they are the same thing. Thereby is a process and therefore indicates a conditional action.

The economy is bad right now, therefore I am going to cut my spending.

The economy is bad right now, thereby dropping the value of my portfolio.

Therefore = “and because of that, _____”
Thereby = “doing it like that, _____”

Therefore answers “Why?”. Thereby answers “How?”.

You can substitute “thus” for “therefore”.

The economy is bad right now. Thus I am on my way to debtor’s prison.

You can’t substitute “thus” for thereby.”

I luuuurve all your answers and have printed them off and stuck them above my desk for future essay reference..thank you kindly xx

…therefore proving that, once again, fluther (on his seahorse) rides in to the rescue.

Wiktionary?

Wat?

wat (plural wats)

Buddhist temple, e.g., Angkor Wat.

From looking at dictionary definitions and example sentences from dictionaries and concordancers (and this website), I am coming to the conclusion that ‘thereby’ introduces the practical result of an action and ‘therefore’ introduces the logical result of an action or situation.

Having said that, it seems to me that the words are often interchangeable. Look at the following example sentences taken from dictionaries and online concordancers. I think both words could be used in these:

1. The aim of the military action was to open the roads to Sarajevo and thereby end the capital’s 40-month seige.
2. In the circumstances the defendants had done that which was reasonably practicable and had thereby satisfied the duty of care imposed upon them.
3. He was injured and therefore unable to play.
4. Dutch euthanasia practice is legalised and therefore becomes a collective responsibility.

Note: I think you could easily substitute ‘thus’ for both ‘therefore’ and thereby’ in these sentences, so I have to disagree with gailcalled’s point.

However, I have noticed that ‘thereby’ is frequently used the -ing form of the verb and ‘therefore’ is not. e.g.:
Our bodies can sweat, thereby losing heat by evaporation.
Jon paid the registration, thereby automatically renewing his membership.

Actually, I think Mritty’s response probably sums up the difference in meaning best: how vs why.

@MarkT: Both #1 and #2 may be grammatically correct but they sound clumsy. I’d substitute “therefore” in both cases. But they may soon be interchangeable, adverb-wise. Ugh.