Question: After reviewing several texts about the second law of thermodynamics and the concept of entropy, from a systemic point of view, it seems to me that entropy is only a circular reasoning concept.

Thermodynamics raised from the study of gases, and gases can't exemplify all types of particles behavior. Gas particles only exert rejection towards others. The main issue is that **natural particles could exert attraction or rejection during an interaction: they don't only exert rejection**. Water molecules, several fundamental particles or simply magnetized particles create attraction forces when they interact. And that is not an isolated natural behavior, it is a very common one. 

In consequence, whatever the concept of entropy is, the second law only expresses a circular reasoning idea: On systems that tend to dissipation, dissipation is more probable. True, but stupid, because *on systems that tend to organization, dissipation is less probable along time*. 

Let's take the case of magnetized particles (as several natural particles behave) inside an isolated space: after some small time, particles will group.

1. If entropy=chaos, entropy decreases with time.
2. If entropy=membership of a small group of states [see: Daniel Styer, insight into entropy], then, entropy also tends to decrease: small groups happen more often.
3. If entropy=energy dissipation, the same. If particles exert rejection towards others, entropy grows with time. If particles exert attraction, entropy decreases with time. In addition, energy is a subjective concept. An object that is heat for a subject (and therefore can exert work) can be cold for another (not being able to exert work), so energy dissipation becomes a subjective approach.

In final terms, **the root problem of the 2nd law is this: it formulates a law that is applicable only to systems that follow that law**.

So, can the 2nd law of thermodynamics be a circular argument?

Answer: 
The 2nd law of thermodynamics is an assertion. It is a statement that someone made that they believe to be true. It is also an assertion that is backed by an enormous body of evidence so vast that the prevailing belief is that no counter example exists. We have never observed a closed system in which entropy does not increase, nor have we observed any almost-closed systems that exhibit a sufficient decrease in entropy to cause us to assume the results were caused by the 2nd law failing. It is considered far more likely that the imperfections in the close system were the cause of the results.

Can we prove it? Of course not. Science does not prove anything. That's not its job, never has been. Science's job is to create empirical models of the world which are very effective at predicting what will happen. This happens to be a model which has been empirically validated countless times. However, there's no reason there couldn't be a single pocket of the universe somewhere where the 2nd law doesn't apply. We just haven't seen one, and we currently have no reason to believe there are pockets where the rules are different.

As for the magnetic particles, our models predict that they will indeed follow the second law. Because the system is isolated, energy does not get in or out. Thus, as the particles become attracted, they cannot emit energy outside of the system. They will acquire a high velocity. Quickly their path will be unpredictable, so the entropy of the system will increase. Eventually, if they collide and their collisions are not elastic, some of that magnetic potential energy you put into the system at the start will be converted to the random movements of thermal energy. Eventually, the particles may finally form a "group," but they will do so with a thermal energy that causes them to vibrate in a statistical manner. This is where you find the loss of order. When you started, the velocities of your particles was known, well ordered. Now the velocities are less known, more disordered.

Again, feel free to test this yourself. Science can never prove itself right. But that's what the models say will happen, and some of those models have withstood quite a beating from the scientific community seeking exotic circumstances where their laws might not apply.
Thanks for your answers, I think I have the answer. The problem was trying to find a relation of proportionality between entropy and any of the presented features.

But I realize now that entropy sustains the 2nd law, and viceversa. In simple words, there is some physically measurable quantity (entropy) that always increases (2nd law) on close systems evolving spontaneously. This, independent of order, enthalpy, and even probabilities. A system can get order, evolve towards a particular state out of the median states, or even concentrate its energy, while entropy reaches a maximum.