The inability to define certain things has ramifications beyond esoteric discussions of the divine chariot and the end of days.
The expression, “the heart cannot reveal to the mouth” (Ecclesiastes Rabba 12:10), appears in connection with all sorts of subjects, for not everything that a person thinks can be expressed easily in words.
There also exists a much more complex and difficult situation, when “the heart cannot reveal to the heart,” that is, that the heart cannot reveal even to itself. These are difficulties that every person experiences at one point or another in his lifetime.
The Talmud (Pesahim 54b) presents a list of things that are concealed from us: the day of a persons death; the day of consolation; the full depth of justice; that which is in another person’s heart – and the list goes on.
The connection between these things is that they are all impossible to determine.
Why is it impossible to know what is in another person’s heart?
Because everything that a person draws from deep inside him he must communicate through an intermediary mechanism, the translation from thoughts and feelings into words.
The listener then transfers the matter from those words into his own heart.
My contact with another person’s heart is, at best, twice removed from the source; there is no possibility of direct contact, of one spirit truly connecting with another.
–Rabbi Adin Steinsaltz