Like "say," the word "speak" refers to thoughts and feelings moving from our more internal spiritual levels to our more external ones -- and ultimately from the Lord, who is in a sense the most internal spiritual level of all. This is generally called "influx" and "perception" in the Writings, meaning they are thoughts and feelings that flow in in a complete way from the Lord, rather than being things we have to think about and figure out. On a number of occasions "speak" and "say" are used together; in these cases "speak" refers more to intellectual instruction in matters of thought and "say" refers more to feelings and affections that flow in directly.

19. 17. To whom will ye liken Me, to whom am I equal? saith the Holy One. Lift up your eyes on high, and behold, who hath created these things, etc. (Isaiah 40:25-26).

(My judgment is with Jehovah, and the price of My work is with my God. Then shall I be precious in the eyes of Jehovah, and my God shall be my strength. Jehovah hath forsaken me, and the Lord hath forgotten me (Isaiah 49:4-5, 14). Because of Jehovah that is faithful and the Holy One of Israel who chooseth thee [Isaiah 49:7].)

I have given Thee for a light to the nations, that Thou mayest be My salvation unto the end of the earth. I have given Thee for a covenant of the people, to restore the earth, to divide the devastated heritages; to say to the prisoners, Go forth; to them that are in darkness, Reveal. They shall feed in the ways, and their pasture shall be on all hillsides; they shall not hunger nor thirst; by the springs of water shall He lead them (Isaiah 49:6, 8-10).

(Let him trust in the name of Jehovah, and stay upon his God (Isaiah 50:10).)

Look to me ye that follow after justice, ye that seek Jehovah; look unto the rock whence ye are hewn (Isaiah 51:1). Awake, awake, put on strength, O arm of Jehovah, awake as in the days of old (Isaiah 51:9).

I will put my words in thy mouth, for the planting of the heavens, and laying the foundations of the earth (Isaiah 51:16). (My people shall know My name in that day, for I am He that doth speak: Behold, it is I. How delightful upon the mountains are the feet of Him that bringeth good tidings, that causeth to hear peace, that saith unto Zion, Thy king reigneth. Watchmen shall lift up the voice; together shall they sing when they shall see eye to eye, that Jehovah returneth to Zion (Isaiah 52:6-7).

Jehovah hath made bare the arm of His holiness in the eyes of all the nations; and all the ends of the earth have seen the salvation of our God (Isaiah 52:10).

Who hath believed our word that is heard? and upon whom hath the arm of Jehovah been revealed (Isaiah 53:1)?

(The Lord is treated of throughout the chapter, and the state of His life is described here briefly in these words. That He had no form nor honor; that He was despised and not esteemed. That He was wounded on account of our transgressions, and bruised on account of our iniquities. That Jehovah hath made to light on Him the iniquity of us all. That He was brought as a lamb to the slaughter; that He was cut off from the land of the living. That because He offered His soul as a guilt offering His days were prolonged, and that the will of Jehovah prospered by His hand, in the fact that He bore their iniquities, and for them poured out His soul even unto death. That He was numbered with the transgressors, and interceded for the transgressors (Isaiah 53:1-12).