The truth, in summary
G. H. Wilson, commenting that the first Psalm is an introduction to the entire book of Psalms:
‘If you were to open a handwritten medieval manuscript of the Psalms at its beginning, chances are that you would discover this psalm—the first in the canonical collection—written in red ink and without any evidence of a number. That is because at an early date the psalm we now know as Psalm 1 was understood to be an introduction to the whole Psalter rather than just another psalm. It is likely that the final editors of the Psalter chose Psalm 1 as the gateway to the psalms because it encourages the readers/hearers to consider the songs that follow to have the effect of divine guidance or torah. This psalm also exhorts the readers both to read the psalms and to meditate deeply on the message God is communicating through them. It strongly affirms that how one responds to the revelation of God, unleashed by reading the psalms, determines one’s ultimate destiny’.
(G H Wilson, Psalms (Vol. 1, p. 92). 2000, Zondervan).