St. Teresa of Avila, in explaing how one experienced contemplative prayer, or the encounter with Jesus, wrote that it was like someone who was blind or in darkness and a person is with them. They can speak to that person and they know with certainty that that person is there with them, but they do not see the other. That is how it was with her when she thought of our Lord [The Book of Her Life, Ch 9:6].
I often describe it as like when you are sitting in Church with your eyes closed, you know when someone has come into the same pew as you. Or like a child coming into his house after school, and just knows that his mother is not home and the house is empty - or the othere way around, knows his mother is home but he cannot see her.
Sometimes Saint Teresa would feel the presence of Christ come upon her and she had no doubt that He was within her and that she was totally immersed in Him. She clarified that this was not a vision. The will loves. The memory or intellect was not working, and yet, it was amazed at all that it understood, because the Lord gave the soul understanding [Ch10:1].
Sister Mary Matthew refers this as being "Devinely occupied".
In another place St. Teresa describes it as like a swoon, the breathing and the bodily energies gradually fail. You cannot lift your hands without effort. The eyes close without you wanting them to. You hear but don't understand what you are hearing. Thus the senses give you no help. You may try to speak, but no words form. "All the external energy is lost, and that of the soul is increased so that it might better enjoy its glory. The exterior delight that is felt is great and very distinct" [Ch 18:14].
Teresa of Avila, The Book of Her Life, trans. Kieran Kavanaugh and Otilio Rodriguez (Washington DC: ICS Publications, 1976).