The philosophy consist in our thinking about something.
The things we think of, come from our being in the phenomenal world – things we notice through our being in phenomenal world.
Not all things we notice can be subject matter of philosophy (by philosophy, I will be thinking of metaphysics here mainly). Specifically the thoughts of particulars can’t be subject matter of philosophy. It can’t be subject matter of philosophy how many people are there on this planet today, or what is the color of the hat of that person I saw today, and so on. As long as philosophy is quest for universal truths, the subject matter of philosophy can be only universals.
What can we hope in philosophy in relation to those universals? When can philosophy be happy with its treatment of those universals?
Philosophy, its life, and its end can’t be in any other form, then form of thought.
The mere noticing of a universal is for sure not what philosophy is about. This noticing is obviously required, as if we don’t notice a universal we can’t even start to think about it, but the philosophical thought doesn’t stop at noticing, it tries to go further in the attempt to grasp the essence. But what kind of form this “grasping the essence” of a universal can take? It seems to me, it can’t be in any other way but through understanding, which in turn can be only a comprehension of its relations to other universals.
As long we are hoping for grasping, or comprehension, this can’t be empirical matter, or be settled by empirical means. The empirical data can merely point to, or help us give a theory about the relation between universals (as for example science does). But, this is not comprehending, those relations are left in such way as something external to the universals and are merely one new universal (a law, theory, etc..) which we add. This kind of outcome is obvious in the latest developments of science, where the laws and theories are said to go outside of the possibility for intuitive understanding.
If the understanding is some kind of thought which includes relations between multiple universals, what can be say about the form of any final comprehension (or comprehensions) which we might hope to achieve? It seems to me, to be seen as comprehension, the relation must be grasped as a necessary one. But then, where can a necessary relation between those universals come from?

I will stop for the moment with the general talk here, and concentrate on one example of such comprehension, and further argue that the last question is not a good one.
The example I’m thinking of is the basic comprehension that one and one make two.
I won’t put it in a form of a separate sentence (“one and one make two”), as I don’t want to frame the discussion in the terms of comprehending a truth of the sentence (or proposition) and I believe that one and one is two can be grasped without having words for those universals. One might ask how can one think of a universal without having a word for it? But to answer with a question – how can one learn the word for a universal without noticing the universal in first place? And when one has noticed the universal, it is a possible content of that person’s thoughts.
Nor will I put it in form of mathematical formalism (“1+1=2”), because comprehending that one and one make two is done everyday by young children before they learn any mathematical signs or formalisms.
And those who might say that what I said is not precise enough, and that they can make a complications, in which one and one won’t be two, I will just ask to ignore those possibilities. I’m thinking of the case without those complications. To say that one can introduce complications, is to admit that before the complications there is something clear.
How do one come to that comprehension that one and one make two?
We can start by ostensive teaching, where we show sequence of situations to the student . First we show one apple to the student, and say “one”, then we show two apples and say “two”, then we show two lemons and say “two”, then we show one pen and say “one” and so on. The situations are concrete, but what is required from the student is to notice the universals in those situations, and to start to recognize them. The use of words can be seen merely as a pointer, a help that student can use through the training to check if some universal he/she noticed is the one to which we are pointing to. For example the student might first notice that the examples have different colors, but after we use “one” and “two”, and the color of the exemplars don’t change, student doesn’t have reason to think that we are pointing(teaching) to the colors, the student will then start searching for another universal in the situations.
After noticing those universals and learning to recognize them, student can think about them. The student can comprehend that one and one make two. As said, this comprehension is not of a linguistic nature, even the language is used to train the subject (to point to the universals). The student might have noticed the universals one and two even without words for them (actually student did notice them before learning of the word, and needed to further check the repeated example in order to figure out if that is the universal the teacher is pointing to). The student can also comprehend that one and one make two without the language to express that relation. The comprehension has nothing to do with the method of learning, nor with the language, as long as one can think of those universals, one has the ability to comprehend that relation.

Now, let’s return to the question asked – where does this necessary relation between the universals come from?
There are, as far as I know, two basic accounts. The first is what I will call an “engineering account”, where we start to talk about the nature of the concepts and how they are grounded in the underlying capacities of the mind/brain. This engineering account tries to give a theory of the mind (or the brain), including in this theory a theory of learning of concepts, forming thoughts, figuring out relations and so on, with the purpose of explaining the comprehension. Or, said otherwise, the comprehension serves as a pointer to a necessary structure of the mind.
A most prominent example of engineering approach is Kant’s philosophy. Giving to these comprehensions the name of “synthetic a priori judgments”, Kant tries to give a theory of necessary structure of mind which would make those (synthetic a priori judgments) possible.
That’s why I call this approach – engineering approach. The situation is similar to one where an engineer is given a set of functions a machine needs to perform, and now the engineer tries to figure out what kind of machine can perform all those functions. The more precise the requirements are, the less choices the engineer has. In the ultimate case, the engineer would not have much choice, but the requirements would necessitate the design of the machine. There would be only one possible machine which would fulfill the requirements (Add another requirement, and it might become impossible to design a machine that fulfills all requirements). Modern cognitive science would fall in part also in this kind of approach, with the difference that it also uses empirical method. But eventually, both sides should meet – the idea is that we will ultimately be able to show how specific design is able to perform all those functions.
The second kind of account of those comprehensions, is the formalistic account. The universals are taken as expressible as specific formulas in specific formalism, and the truth which we comprehended, is then stated in that formalism as a proposition, and further it is shown how it follows from the formulas we used for the universals (in this case one and two), and bunch of principles of deduction. The example of this kind of account is the one which was started by Frege, and then continued by Russell and others.
One important difference is that engineering approach tries to give account both of the thing comprehended and the comprehension, while the formalistic account is left merely with explaining the comprehended, and isn’t much interested how the comprehension is possible (even sees the issue of comprehension as being psychological question, and not proper philosophical issue).
But as much those two approaches are different, both have one thing in common, that they think that there is a need to go deeper than comprehension. One could say that both approaches “feel” that there is something underlying the comprehension, some kind of form in which the universals are actualized, and that it is because of the nature of this form, that there is a necessary relation between the universals.

But, why should we go deeper? What are we hoping to find? What is that, which is more than clear comprehension of relations between universals? Do we want to do away with the universals? Reduce everything to different forms in one essence?
Much more could be said on this, but this post is already very long, so let me finish with one quote of Hegel:
What we are dealing with in logic is not a thinking about something which exists independently as a base for our thinking and apart from it, nor forms which are supposed to provide mere signs or distinguishing marks of truth; on the contrary, the necessary forms and self-consciousness of thought are the content and the ultimate truth itself.
What should be kept in mind when reading this Hegel’s quote is that by “logic” he doesn’t mean formal logic but the clear thinking of the universals in their relations. Universals as both something thinkable, and something in the world. Hegel’s Logic (Science of Logic), is this attempt then to grasp the relations between the universals, not from outside by translating them as concepts to some form in which we ourselves have set the conditions of what makes a concept, a thought, and what makes the thinking valid and true, and in which we can then treat them as a mere form, but to comprehend the relation of the universals through what they are in themselves, and how they relate between each other in themselves.
Long live metaphysics!