Question

1. I think the telephone is the most important invention.

2. I think a telephone is the most important invention.

3. I think telephones are the most important inventions.
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Are they all the same in meaning?
Can we use only #1 as generic use of 'the telephone'? Is 'a telephone' or 'telephones' in generic use as well?

Answer 1

Yes, they mean the same thing, but #1 is best, most grammatical. "A" telephone? There are more than one? "The" telephone means the invention, not a specific telephone. And #3, using plurals, again implies that there were more than one inventions of telephones. There was only one invention (although it has been refined and changed over time).