That is why St. Paul, knowing better than we the great blessing that this prayer brings to us, commanded us to pray unceasingly [1 Thess. 5.17]. He would not have bound us by such an obligation if it had been extremely difficult and impossible, knowing beforehand that in that case, by not being able to perform it, we should inevitably prove disobedient to him and transgressors of his commandment, and should thereby deserve condemnation and punishment. And this could not have been the intention of the apostle.
Besides, take also into consideration the means of prayer, how it is possible to pray unceasingly, namely by praying with the mind. This we can always do if we want to. For even when we are sitting at some manual work, or what we walk, or take food, or drink, we can always pray with the mind and perform prayer of the mind, pleasing to God, true prayer.
Let us work with our body and pray with our soul. Let our outward man perform his bodily labors and our inward man be consecrated to the service of God, and never slack up in that spiritual work of the prayer of the mind, as the God-man Jesus also commands us, saying in the Holy gospel: But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father, which is in secret [Matt 6.6]. The closet of our soul is our body; our doors are our five senses. The soul enters into its closet when the mind does not wander to and fro among worldly matters and things, but remains within our heart. Our senses are closed and remain so when we do not allow them to cling to outward sensible things, and in this way our mind remains free from every worldly attachment and through secret prayer of the mind is united with God our Father […]