Quantum mechanics has been successful in understanding nature. It is self-contained, providing self-consistent logics and necessary information of whatever subject. From a practical point of view, there is no problem there. In some extreme cases, however, the logic for relating quantum systems to our cognition is strange at best. From the very start of quantum mechanics there have been troublesome questions about quantum measurements. Our brains, the last device for our cognition, are macroscopic. We have to use a macroscopic device for any measurement of a quantum system, however small it may be. There is a mist between a very small quantum system and a macroscopic device. Furthermore, certain distinguished physicists asserted that we need hidden variables to solve all troublesome problems concerning the measurements of small quantum systems. The assertion is that the current quantum mechanics possesses no practical difficulties but the troubles in its interpretation can only be solved by the hidden variables. All efforts to answer the troublesome questions are classified as the measurement theory. The most serious question is that the conventional interpretation of quantum measurements at issue requires nonlocal events, violating Einstein's hypothesis of prohibiting instantaneous propagation of natural phenomena over the light velocity. A typical example is the Einstein-Podolsky-Rosen experiment, which might have favored the hidden variable theory. The question was greatly clarified by John Bell who found an empirical criterion for the existence of hidden variables by means of mathematical inequalities known as Bell's inequalities. The experiments along this line have not yet rejected the non-locality.