Anaximander (610 – 546 BCE) was an ancient Greek philosopher under the Milesian School of thought with Thales and Anaximenes.
He believed the one ultimate substance of all things was apeiron, which translates to “boundless”. It’s seen as a formless, infinite substance, that can be built into other substances and broken down back to apeiron as needed. It also is the source of opposites such as hot/cold, wet/dry, etc).