pass through, note, commentary, estrangement, sparseness, neglect, penetrate
疏 centers on things being apart or not dense: sparseness, estrangement, and neglect all involve distance or looseness. The ideas of passing through, penetrating, and commentary extend from making a way through something sparse or clearing a path.
疏 combines ⺪ (a foot or movement component) with 㐬, which likely contributed the sound and the idea of flowing or clearing. The modern form is associated with sparseness and passing through, but the exact historical development of the components is uncertain.
The left side ⺪ suggests movement, and the right side 㐬 looks like a sparse, open structure. Imagine walking through a sparse forest where the trees are far apart, making it easy to pass through. That sparseness can also mean neglect or estrangement when things are too far apart.
For ソ, picture sewing a sparse fabric: sew -> ソ, and the loose weave lets light pass through easily.