What she means is that the verb "are" (to be) simply means that the object of the verb exists. So the two sentences: 1) "There are various theories concerning this phenomenon" and 2) "Various theories concerning this phenomenon are" are both equivalent to sentence 3) "Various theories concerning this phenomenon exist." In all three cases, the object of the verb is "this phenomenon" so that no ellipsis waiting to be filled in with what the theories are is necessary. The statement is simply that the theories are; that is, that they exist. All three constructions are grammatically correct, which is her point. Saying that one construction is better than another is a statement of preference, but not a statement of grammatical correctness.Moonbear said:I'd add one modification to that:
"Various theories concerning this phenomenon are..."
In other words, the sentence still needs to be completed to state what those theories are. Otherwise, it's a meaningless sentence that just wastes space in a scientific paper. Such a sentence may have a place in other forms of writing, such as English essays. I suppose when a student comes to a "Physics Forum" and asks a question about thesis writing, I'm assuming they mean scientific writing, not a thesis for an English or history degree (although, even in those cases, I'd have a hard time accepting students would not be expected to be precise in their meaning).
It does, however, seem that she is implying that preferring one over another is arbitrary, and simply up to the whim of the professor, which I don't necessarily agree with. Constructions 1 and 3 are not ambiguous, whereas construction 2 can easily be misconstrued, as evidenced by the fact that a biology professor just misconstrued it, as a sentence fragment, rather than a complete sentence, and so should probably be avoided. Yes, you do need to be clear about what it is that you are trying to say, as clear as the english language permits you to be.
Anything beyond that, and beyond actually grammatically correct construction, however, is just the whim of the professor, or more likely of accepted convention, like what Swerve was talking about regarding professional dress codes. In that case, such as distinguishing between whether we use first person singular, first person plural, or passive narrative voices, when engaging in expository writing, it is simply a matter of cultural and social expectations, not of precision or of the correct usage of language.