Regarding the use of "As [it] is the nature of"

What follows is a response to a question about the difference between the opening clause, "As is the nature of..." and, "As it is the nature of...".

"AS" has two meanings in this case depending on context:
1) "As it is the nature" = a clause setting up a cause/effect relationship between a characteristic that is fundamental to the item with an effect that is not fundamental. (See below.)
2) "As is the nature" = a clause defining the fundamental nature of the item in question.

Let's abstract a model-form sentence starting with "As it is the nature": "As it is the nature of A1 [to be in a condition of] X1 , Y1 happens." Here, "it" refers to the implied condition of X1 upon A1, which results in a further condition, Y1.

In this case, X1 is a condition related to the nature of A1, but Y1 is not related directly to the nature of A1. Y1, however, is a condition related to X1. Therefore, the statement "As it is the nature..." in this case sets up a clause that refers to the nature of A1 to X1 with the expected further relation to a nature Y1, but does not say anything about the relationship between A1 and Y1.

Now, let's abstract the other clause: "As is the nature of A2, Y2 happens." Here, Y2 happens because of the fundamental nature of A2. In this case, the fundamental nature of A2 results in Y2. Removing the "it" in the statement removes the implied condition (X1) seen in the previous example, and extends the fundamental definition of the subject (A2) to include the above implied condition (X1) within it.

In other words, A2 = A1 + X1.

Let's look at an example of the use of "As it is the nature of..." I'm quoting here from David Hume's A Treatise of Human Nature.

• "As it is the nature of doubt to cause a variation in the thought, and transport us suddenly from one idea to another, it must of consequence be the occasion of pain."

Abstracting this sentence we get: "As it is the nature of A1 to cause X1, it must do Y1." Here, the condition of X1 ("variation in the thought, and transport us suddenly from one idea to another") upon A1 ("doubt") leads to the consequence of Y1 ("be the occasion of pain").

We can, of course, change the sentence to the "As is the nature..." form by integrating the former condition X1 with the former object A1 thusly:

• "As is the nature of doubt-caused variation in thought transporting us suddenly from one idea to another, pain must be the consequence."

While this is not the best re-rendering, I hope it will illustrate the example. Here, the abstracted sentence structure becomes: "As is the nature of A2, Y2 happens." Here,the fundamental nature of A2 ("doubt-caused variation in thought transporting us suddenly from one idea to another") causes Y2 (pain).

Hope that helps you understand what it's all about.