Rational

Both rational and ratio share the same Latin root.

So if something is rational (sensible, coherent), it takes proportion (ratio, balance, correlations) into account.

Proportion is a relationship between elements. It is often linked to beauty and harmony. It implies a harmonious integration of the parts in a whole.

Thus, true rationality regards both the whole and all of its parts. Rationality is, in this sense, inclusive.

Irrationality at its worst would then mean disregarding a part or several parts of a whole, in favour of others. Deviating visibly from an equilibrium, slanting markedly.

There is, of course, another ingredient to add to this: time. Time usually corrects excessive slanting by turning it around and promoting slanting in the opposite direction.