"... but man, proud man, Drest in a little brief authority, Most ignorant of what he's most assured, His glassy essence, like an angry ape, Plays such fantastic tricks before high heaven As make the angels weep; who, with our spleens, Would all themselves laugh mortal." Here is a quote from Shakespeare's Measure for Measure. What does "glassy essence" mean here?
There's plenty of notes and analysis on pretty much any passage of Shakespeare you could think of online. A quick Google search came up with this:
His "glassy essence" — the fragility of his soul