All dharmas are of the nature of suchness (tathatā)

In the Platform Sutra, the Sixth Patriarch said: 一切诸法若无常者，即物物皆有自性，容受生死，而真常性有不遍之处。故吾说常者，正是佛说真无常义。

Self-nature gives rise to all dharmas, and the essence of all dharmas is also self-nature. Self-nature is permanent, and since all dharmas are spoken of in dependence upon self-nature, they too are permanent. If all dharmas were impermanent, then everything would have its own inherent self-nature, capable of arising and ceasing by itself; they would not be born from self-nature, nor would self-nature be needed to give birth to them. In that case, self-nature would have places it does not pervade. However, this is impossible. Therefore, all dharmas, spoken of in dependence upon self-nature, all possess the permanent nature of suchness self-nature. If it were said that one dharma is impermanent, then that dharma would have its own inherent self-nature, for only with an inherent self-nature can there be talk of birth and death—would not suchness then have places it does not pervade, dharmas it cannot give birth to?

When all dharmas are spoken of in dependence upon suchness self-nature, they are all of the nature of suchness, neither arising nor ceasing. Because these dharmas are all born of suchness self-nature and all possess the nature of suchness, all dharmas are likewise permanent and imperishable. As long as there is suchness self-nature, all dharmas will always appear. If all dharmas were impermanent, then once they ceased they could not arise again; if they could not arise, it would mean that suchness self-nature had ceased to function and could no longer produce dharmas—that would mean suchness self-nature had places it does not pervade. However, this is impossible. Suchness self-nature pervades all places; in every place and every dharma there is suchness self-nature. Wherever there are all dharmas, there is suchness self-nature. Moreover, the seeds inherently present in suchness neither arise nor cease and endure forever; as long as conditions are met, they will give birth to all dharmas.

The meaning of what the Sixth Patriarch said is: if all dharmas were impermanent, then all things would each have their own inherent self-nature, and these things could arise and cease by themselves, in ceaseless arising and ceasing. In that case, suchness self-nature would not be needed to give birth to them, and self-nature would have places it does not pervade—this would contradict the meaning of the sūtras. Therefore, when the Sixth Patriarch said that all dharmas are permanent, this was precisely the true meaning of the impermanence spoken by the Buddha. That all dharmas are permanent shows that all dharmas are born of and manifested by self-nature; thus all dharmas have no inherent self-nature of their own, and therefore they are impermanent.