There have been frequent claims, both from legitimate sources and quackeries, of the possibility of finding something called the Theory of Everything (TOE). Supposedly, such a theory would contain ALL the necessary interactions that would be able to, in principle, describe ALL the phenomena (at least the existing ones) that we have observed.
We won't talk about the quackery aspect of this. The legitimate aspect of the belief in a TOE is due to the fact that there are only 4 fundamental interactions that are responsible for all the phenomena in our universe that we know of: gravity, electromagnetic, strong, and weak interactions. There is a strong belief (and desire) that all 4 of these fundamental interactions can be unified into a single consistent description. Already the electromagnetic and weak interactions have successfully been unified into an electroweak theory. There are every indication that the strong interaction will be next. Gravity might be the last and most difficult. However, assuming that it can be unified with the others, one then have what is called the Grand Unified Theory (GUT). Most people claim that then, we have achieved the TOE.
The implicit assumption in making such a claim that GUT = TOE is that the principle of Reductionism works. Reductionism is a philosophy which, to state rather crudely, everything in the universe can be reduced to the basic interaction at a single particle case. Then, by simply adding a higher level of complexities, one can then recover all the other more complicated, macroscopic phenomena. So if you know all the interactions that an atom in a human skin has, then by including more and more of the number of atoms/molecules, you will eventually be able to describe all the properties of the human skin. Hence, once you know all there is to know at the single particle scale, then everything else is just a matter of complexities. This then leads to the notion that GUT = TOE.
Most particle and high energy physicists espouse this point of view. I would single out Steven Weinberg as the prominent champion of this school of thought. String theoriests have also been known to slip up now and then by claiming that unification of gravity with quantum mechanics is a step towards GUT and TOE.
However, there is another school of thought that would contradict the idea that GUT = TOE. This school of thought is made up of condensed matter physicists, which make up the largest percentage of practicing physicists. The most prominent condensed matter physicists who have stated their opposition to the reductionists idea are Phillip Anderson, Robert Laughlin, and David Pines. They brought up examples that are described as "emergent" phenomena, often seen in condensed matter. These are phenomena that only occurs, or can only be defined, when there are a gazillion interactions occuring. Examples of these are superconductivity, fractional quantum hall effect, magnetism, etc. Laughlin, for example, argued that if you try to write down all the interactions of a single electron in a conductor, no matter how many electrons you add up in your interactions, you will NEVER recover the superconductivity phenomenon. Superconductivity is an emergest phenomenon that is a result of a many-body interaction. The starting point in describing such a phenomenon MUST start not from a single particle scenario, but from a many-body ground state scenario. This effect emergers out of a many-body interaction and will simply disappears if one tries to take it apart to a single-particle level.
What this boils down to is the claim that GUT is the TOE for reductionism, not the TOE for physics. Anyone claiming the existence of any form of TOE will have to seriously address the glaring omission of a huge body of phoenomena from condensed matter physics, which holds some of the most highly verified observations with the highest degree of certainty in any field of physics.
For more resources on emergent phenomena from condensed matter physics and why they contradict the claim of GUT=TOE, read the references below:
4. R.B. Laughlin, Rev. Mod. Phys., v.71, p.863 (1999).
3. P. Anderson, Science v.177,p.4 (1972).