What does "And then again" mean?

Question

... Nothing exists but the sensations and ideas in our minds, and there is no external material reality corresponding to them.

And then again, this subjective idealism is put forward in the form of a doctrine concerning knowledge: it denies that we can know anything about objective reality outside ourselves, and says that we can have knowledge of appearances only and not of "things in themselves".

Materialism and the Dialectical Method (Maurice Cornforth)

What does "And then again" mean? Does the author mean "and then, again" or "and, then again"?

Answer 1

The short answer is that he means "and then, again, this subjective idealism..." He means to add something further and in agreement with his first statement.

Here is my longer answer. Given the context of the excerpt you have provided, I would read what follows "And then again" as adding another layer to the previous statement, a layer not of contradiction but of the same basic philosophical principle applied to a different subject. I would need more of the preceding statement to be absolutely sure, but it seems that "nothing exists but the sensations and ideas in our minds, and there is no external material reality corresponding to them" speaks to a certain view of the nature of perception and being (which makes it an ontological statement). In the next section, Cornforth names this ontological view (subjective idealism) and applies it to the nature of knowledge or knowing (an epistemological statement). "And then again" shows how subjective idealism can be applied to another subject of philosophy, from ontology to epistemology.

Answer 2

"Then again" usually introduces an opposing thought or idea or evidence that contradicts, even slightly, whatever assertion came before.