Peter Pett's Commentary on the Bible
Psalms 1 - Introduction
BOOK ONE (Psalms 1-41).
This psalm is introductory to the whole collection. The entire psalm extols the blessedness of the one who avoids the path of the sinful, and delights in the Instruction (Law) of YHWH, walking in its truth. Such a person chooses the way of righteousness.
The Psalm initially declares what the way of the righteous is by describing what it is not, and this is followed in Psalms 1:2 by an indication of what specifically differentiates the righteous, resulting in Psalms 1:3 in the declaration of their great reward, that their lives flourish and blossom like a tree beside life-giving streams.
In Psalms 1:4 he points out that the way of the unrighteous is the very opposite of that. For instead of being firmly rooted they are swept away as the chaff is swept away by the wind, with the result that, in contrast with the righteous they will be unable to face God when He judges (Psalms 1:5). Psalms 1:6 then summarises the situation, explaining that the way of the righteous is known to God, while the way of the unrighteous perishes. It is the righteous who truly live.