Heaven and Earth are ruthless; to them the Ten Thousand Things are but as straw dogs. The sage too is ruthless; to him the people are as but straw dogs.
(Laozi, 5; Waley)
This passage is powerful if for no other reason than for its shock value. Sometimes it takes the shock of the extreme to clarify the center. The center is clarified when it simultaneously embraces the extremes. For the extremes are in fact 'true', but only in the context of each other.
Nature is indeed ruthless, but not by way of intention; it is simply the nature of the case that things arise, flourish, decay and pass without any apparent consideration of our desire that it be otherwise. They have no apparent value apart from the event of their arising and passing, but there is value in this, if we are willing to let it be so.
Straw dogs were effigies made of straw, assigned significant ritualistic value, and were then discarded as the valueless straw that they actually were. They came into being, had their valued moment, and returned to the emptiness and valuelessness from which they came — just like everything else, just like you and I.
Treating people as straw dogs is thus to value them greatly during their moment in the sun, yet letting them go when their time has come. Their value subsists in that they exist, but they exist by virtue of arising from the valueless to which they return. So, the valueless is what gives value. This is the usefulness of the useless. So let us value the valueless and the useless. This is the ironic Dao — that which gives value, but itself has no discernible value.
To speak of Mystery as valueless is, of course, to err if we take valuelessness as the literal extreme of the valued. We call it valueless only because we are unable to fathom its value; but this is not to say that it has a value, though we cannot know it; value and its lack are merely the creations of straw dogs; what has Mystery to do with our momentary need for value?
Being, as we are, straw dogs, however, we do experience a need for value and meaning. We are alive, not dead. Thus, we value life and find it meaningful. But value and meaning, if they are to be "sustainable", seem to require an unshakeable foundation, and thus we posit one. But the Dao that can be posited, the foundation that is conceived and articulated, is a chimera; it not the "sustainable Dao". What Dao is sustainable? That Dao that is no knowable Dao, that value which has no known value, that meaning that is essentially meaningless, these are sustainable. Not-knowing is sustainable. The incomprehensible is sustainable because it is (or is not) whatever it is (or is not) without our mediation in declaring it so. We rely on this, not an idea thought up. True not-knowing is ultimately a mystical experience of letting go into Mystery, of allowing that value and meaning are grounded in what provides them no ground. And thus, “The Radiance of Drift and Doubt is the sage’s only map.” (Zhuangzi 2:29; Ziporyn)
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