Hebrews lets us know that solid food is for the mature, whose spiritua senses perceive or know heavenly matters. These are they who have been adequately trained by what they've experienced to emerge with understanding of the difference between what is truly excellent and what is evil and harmful (Hebrews 5:15 TPT). So discernment is haveing trained senses (seeing, hearing, smelling, feeling and selling) to what is Good and of God and what is evil and not God.