Wordwizard

1) The theory has to be simple enough to be worth having. That is, it must predict some things that are not in the theory itself (otherwise it is just a list of facts). But neither can the theory be so simple that it cannot explain things it should.

Source:
What’s universal grammar? Evidence rebuts Chomsky’s theory of language
learning
By Paul Ibbotson and Michael Tomasello


http://cogsys.sites.olt.ubc.ca...99s-theory-of-la.pdf

Is the last sentence grammatical?

There seems to me that there is no reason to use 'neither'. I can understand the sentenc, but to me 'neither' sounds incorrect there.

Could one replace 'neither' with 'nor'?

Gratefully,
Navi

The pairing of "neither / nor" to describe phenomena in two or more negative terms is a familiar one, e.g.

The government's decision is neither fair nor sensible.

The president was neither sharp, nor well-informed, nor given to acting in good faith.

However, either word can be used to introduce a follow-on negating idea when it is employed singly, as with your specimen sentence above:

[But] neither can the theory be so simple that it cannot explain things it should.

[But] nor can the theory be so simple that it cannot explain things it should.

(The terms in square brackets are optional.)

Erik is spot on.
Some simpler examples:

• I'm not deaf; neither am I stupid.
• I'm not deaf; nor am I stupid.
• It's not in the kitchen; neither is it in the lounge.

The construction is often used over two sentences. I chose to use a semicolon instead. I could also have used a dash. Whatever punctuation I use, in speech there is a significant pause before the second clause.