Of paths, the Eightfold Path is best, Of truths, the four statements are best; Of qualities, dispassion is the best, Of two-legged beings, the one with eyes (clear vision) is best.

– DhammaPada Verse 273

The picture is showing a garden hose (on the left) that has mud stuck inside as producing muddy water even when clean water is passed through one end (when one is starting out in applying the noble eightfold path). Slowly but surely, as one continues passing the clean water, the mud that is stuck inside (unwholesome mental qualities emerging from the deep-rooted defilements) gets cleared out and the garden hose also produces clean water (on the right).

Related Teachings:

The noble eightfold path is a comprehensive collection of training guidelines that one should practice for the ending of the unwholesome mental qualities and for the benefits of enlightened mental qualities shine through.

Analysis of the Eightfold Path (SN 45.8) - The Buddha presents the eightfold path together with a detailed analysis of each factor. It should be assumed that these explanations apply wherever the eightfold path is taught.

EightFold Path and the Teaching Link / Training Guideline