Heraclitus of Ephesus(535–475 BC) is another pre-Socratic philosopher whose works did not fully survive to the modern era, but some fragmented ideas still remain. He came to be known as "The Obscure," "The Riddler," and "The Weeping Philosopher" due to the seemingly muddled and contradictory nature of his philosophy. But through a modern lens, his ideas are not as obscure as they might once have been.
One of Heraclitus' surviving quotations is Panta rhei, "everything flows." A supporting quote which also survived is "Ever-newer waters flow on those who step into the same rivers." This is a rather succinct was of saying that change is inevitable. The nature of the universe is to continually move through forms that it has not taken before and will not take again. This concept is also known as "flux."
Like Thales, Heraclitus had his own idea for what the quintessential substance of the universe is. For him it was fire. "This world, which is the same for all, no one of gods or men has made. But it always was and will be: an ever-living fire, with measures of it kindling, and measures going out." Heraclitus likely chose fire because of it's wild and destructive nature. Fire destroys and gives birth to new things simultaneously. This frames the flux of the world as wild and chaotic, rather than serene and gradual, as water would be.
Rather than lament this conception of the world, Heraclitus embraced it. Another of his surviving quotes is "strife is justice." "We must know that war is common to all and strife is justice, and that all things come into being through strife necessarily." To Heraclitus, strife was the continual process of being built up and broken down at the same time. Justice is finding a stable state within these dual processes. I find this to be extremely insightful. Without the violent and disorderly nature of the universe, life itself would probably not even exist. If so, is it not reasonable to praise the chaotic nature of the universe for allowing us to come into existence? This is why Heraclitus is known as the weeping philosopher. He viewed the world as a necessarily hostile and forever changing place, but it's still the only world we have. "The fairest universe is but a heap of rubbish, piled up at random."