13 Be not ashamed to confess your ignorance, since you share ignorance with Angels. Only He who begot knows Him who was begotten, and He who is begotten of Him knows Him who begot. He who begot knows what He begot: and the Scriptures also testify that He who was begotten is God. For as the Father has life in Himself, so also has He given to the Son to have life in Himself; and, that all men should honour the Son, even as they honour the Father; and, as the Father quickens whom He will, even so the Son quickens whom He will. Neither He who begot suffered any loss, nor is anything lacking to Him who was begotten (I know that I have said these things many times, but it is for your safety that they are said so often): neither has He who begot, a Father, nor He who was begotten, a brother.
Neither was He who begot changed into the Son, nor did He who was begotten become the Father. Of One Only Father there is One Only-begotten Son: neither two Unbegotten, nor two Only-begotten; but One Father, Unbegotten (for He is Unbegotten who has no father); and One Son, eternally begotten of the Father; begotten not in time, but before all ages; not increased by advancement, but begotten that which He now is.
Source: Catechetical Lectures (New Advent)