 a number of current opinions about the neutral $K$-mesons. They mainly reproduce part of the judgments stated in the works by Gell-Mann [@4] and Gell-Mann and Pais [@5] with adding some corrections for the results of the subsequent experiments concerning the violation of $CP$-symmetry.

Starting from the assumption of strict conservation of the isotopic spin in strong interactions, Gell-Mann [@4] concludes that, among the two long-lived neutral particles produced in the collision of the $\pi^{-}$meson with the proton, one particle ($K^{0}$) is a boson with the isospin 1/2 and its projection -1/2 and that there exists an antiparticle ($\bar{K}^{0}$) with the isospin projection +1/2 which corresponds to the boson $K^{0}$ and is different from it. The mentioned assumption of Gell-Mann is the key element in the subsequently formed structure of the family of neutral $K$-mesons.

Gell-Mann and Pais [@5] consider that, if there exists the decay $K^{0} \rightarrow \pi ^{+}+\pi^{-}$, then there should exist the charge-conjugate process $\bar{K}^{0} \rightarrow \pi ^{+}+\pi^{-}$, and thus, the weak interaction causes the virtual transition $K^{0} \rightleftharpoons \pi^{+}+\pi^{-} \rightleftharpoons \bar{K}^{0}$. The last judgment and the aspiration for providing the $C$-parity conservation in weak decays lead to the introduction of the quanta $K_{1}^{0}$ and $K_{2}^{0}$, which fields are expressed in the form of normalized sum and difference of the fields $K^{0}$ and $\bar{K}^{0}$, respectively. According to Gell-Mann and Pais, $K^{0}$ and $\bar{K}^{0}$ are the primary objects in production phenomena, whereas the decay process is best described in terms of $K_{1}^{0}$ and $K_{2}^{0}$. Each of the latter can be assigned a definite lifetime, that is not true to the $K^{0}$ and $\bar{K}^{0}$.

Note that a long-lived neutral boson produced, for example, in the collision of a $\pi^{-}$-meson with a proton, cannot make any experimental manifestation between the procuction moment and the decay moment and, consequently, it does 