'Thorn and thistle' are used to mean curse and vastation. 'He was to eat the plant of the field' means that he was to live as a wild animal does. A person lives as a wild animal when the internal man is so separated from the external that it operates into it only in a very general way. For what comes from the Lord by way of the internal man makes a person human, but what comes from the external, which when separated from the internal within him is nothing but a wild animal, makes a person a wild animal. The disposition within him is similar to that of a wild animal, as are the desires, the appetites, the delusions, and the sensations. His organic parts are similar too. His ability however to reason, and as it seems to him, to do so incisively, is the product of spiritual substance, by way of which the Lord's life is able to flow into him, but which is corrupted with that kind of person and becomes a life of evil which is 'death'. This is why he is called a 'dead' man.