Innocence is being pure and not tainted by evil.
You can lose something that is not tangible by losing your view on life, mentality, or a character trait you have. Once you lose it, you can never get it back. I connected this to the quote "Man's mind, once stretched by a new idea, never regains its original dimensions." This connects this idea because the dimensions of your brain are intangible but yet they can change.
The woman to the right is trying to cover the naked woman who is losing her innocence.
The woman in the middle is walking away from a sea shell that represents innocence.
The central figure is surrounded by darkness and the darkness represents things that make her lose her innocence.