In the "Divine and Local Simplicity" essay, I left open a question, that if fundamentally thinking and feeling are identical, how did it come about that we now distinguish them? I do not have a satisfactory answer to this question, but I think that the answer is tied to the answer to another question: how do referential forms come about, which is to say, how did language come about?
Recall that in that section I distinguished between mathematical thoughts, and, say, the thought of a house, where the difference is that, for example, the thought of a triangle is the triangle, while the thought of a house is not the house. The thought of a house refers beyond itself, while the thought of a triangle does not. The former is referential, and the latter is non-referential. A referential form points to something else, which could be another referential form, a non-referential form, or formlessness. Excluding the last case, what we have is that one form "brings to mind" another. This implies that the second could lie outside of mind. Assuming that nothing is outside of Ultimate Mind, this could only happen in a localized consciousness such as ours. Thus the need for referential form is correlated with the localization of consciousness.
Sense perception is also correlated with localization, and it is not that hard to see that sense perception is by and large a language (or many languages), that is, is referential. It is generally accepted, once one has moved beyond naive realism, that the objects of perception are referential, although the more commonly used term is that they are 'representations'. Even, or perhaps especially, the critical realist must acknowledge that what is actually seen is not what is "really there". Instead, according to the materialist sort of critical realist, what is actually seen is a product of electro-chemical activity of the brain. For the idealist, what is actually seen is the result of a "meeting of minds", the mind of the perceiver, and the mind whose conscious activity is communicated to the perceiver through what is perceived.
There is, however, a broad range of "readability" in sense perception. Words, assuming one knows the language, are obviously "read through" -- it takes an effort to actually notice them as sense objects. Rocks, on the other hand, if one is not a nature mystic, are not read through at all. But to return to the present topic (the relation between thinking and feeling), I think it is instructive to consider music. "Music is feeling, then, not sound" says Peter Quince in Wallace Stevens' poem Peter Quince at the Clavier. We "read through" the sounds and in doing so are moved, to what we might call sadness or to joy. However, the word "sadness" does not quite make sense, for we may greatly enjoy hearing a slow, minor key piece. Music, one might say, gives us the pure emotion without the bad stuff.
On the other hand, it has been known since Pythagoras that music is highly structured. It can be analyzed mathematically. This makes it, as I see it, also expression of pure thought. And so, one could say that a pure thought is a pure feeling, and a pure feeling is a pure thought. How, then, did impurity arise?
In a localized consciousness, as noted above, not everything is "in mind", and so we need referential forms to bring what is not in mind into mind. This means that a referential form carries an additional purpose than just "being", however lovely it may be, in the way that a mathematical object or a musical passage "just is". And so, we can distinguish the referral to another form from the feeling of the form. That referral to another form is a thought.
As said at the start, this does not solve the question of how thought and feeling became distinguished. It just states that this question is tied to the question of how localized consciousness came about.