St. Isaac the Syrian teaches that there are three kinds of knowledge, which correspond to the division: body, soul and spirit. Naturally, this is not the so called tricomposite of man, because the spirit is not a particular energy of man, but it is the Holy Spirit. Just as the soul is the life of the body, so also the Holy Spirit is the life of the soul. Thus there is the bodily knowledge, the knowledge of the soul and the spiritual knowledge.
B o d i l y k n o w l e d g e is closely connected with the study of human wisdom and knowledge; with the desire of the flesh, the satisfaction of the passions of pleasure-loving, greed and ambition. It is human knowledge, because it deals with the invention and cultivation of the arts, sciences and learning. It is the knowledge which characterises all of our education. St. Isaac emphasises that the partial cultivation of this knowledge creates fear and disturbance, sadness and despair, cowardice before men, dependence on reason and the human powers, fear of death and of the demons. It is a knowledge which makes man appear strong, in relation to his fellow-men, but essentially weak in relation to God and even to the devil. It is the knowledge which is also cultivated today and causes anxiety and insecurity, wickedness and obstinacy.
The k n o w l e d g e o f t h e s o u l is granted when man ceases giving great importance to reason and is engaged in the implementation of the commandments of Christ. Fasting, prayer, charity, the reading of the holy Scriptures, the acquisition of virtues, the fight and struggle against passions are all a result of this knowledge. The Holy Spirit perfects this knowledge, but the co-operation of man is also needed for his freedom to be manifest.
S p i r i t u a l k n o w l e d g e is the state of spiritual theoria, when one sees invisibly and hears inaudibly and comprehends incomprehensibly the glory of God. Precisely then comprehension ceases and, what is more, he understands that he does not understand. Within the vision of the uncreated Light man also sees angels and Saints and, in general, he experiences communion with the angels and the Saints. He is then certain that resurrection exists. This is the spiritual knowledge which all the holy Prophets, the Apostles, Martyrs, ascetics and all the Saints of the Church had. The teachings of the Saints are an offspring of this spiritual knowledge. And, naturally, as we said earlier, spiritual knowledge is a fruit of the vision of God.
-You mentioned previously that St. Isaac distinguishes natural knowledge from spiritual knowledge. He accepts, therefore, two kinds of knowledge. A little while ago you analysed the three kinds of knowledge again according to St. Isaac. Do not you see a contradiction at this point?
-It is only a superficial contradiction. In fact there is no contradiction. For, when St. Isaac speaks of natural knowledge he means the knowledge of the soul which is the acceptance of the Revelation of the Saints and simultaneously the realisation of the works this acceptance entails, i.e. faith based on hearing. In other words, when man accepts the teaching of Christ and keeps it in his life, the knowledge of the soul is begotten. Spiritual knowledge however is associated with theoria. St. Isaac, then, simply adds the bodily knowledge to his second analysis concerning knowledge. Thus, his two distinctions of knowledge are connected and identified at their two points
Metropolitan Hierotheos of Nafpaktos