I know it seems very simple-and it is.
What does and it is mean in this context?
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The sentence you reference uses its two halves to distinguish between appearance and the truth. Since things can seem one thing but factually be another, pointing out that the it is truly "very simple" is useful.
The "and it is" portion of the sentence sort of answers an explicit question:
I know it seems very simple
This sentence introduces elements of doubts, using the work "seem" and presenting it as one persons thoughts. Because of this, it sounds like the following clause will go something like "but it is actually pretty difficult".
Clarifying that the way something seems and the way it actually is are one and the same is important. Therefore adding a shortened version of "and it is very simple" is important and almost necessary so the reader is not confused. Therefore (at least in my understanding) the sentence would be read as:
I know it seems very simple is it actually simple? - and it is [actually simple]
With the superscript being the thoughts of the reader.
This premise is further reinforced by the common phrase "Things are not always as they seem", therefore when things are as they seem, it is sometimes significant to point that out.