The separation of powers means that the people who judge according to the law aren't the ones who set or apply the law, and the people who apply the law aren't the same ones who set the law, so neither can act arbitrarily. The police may disagree with the judges on an unjust decision, and the judges may disagree with the lawgivers on an unjust law. Checks and balances.
What the founding fathers did not realize is that the people who set the law should also not be the ones who choose the law, because that reduces to the same point: whatever the law the government sets is automatically applied because the government also sets the judges and the police, so they might as well be it themselves and the separation of powers does not work.
Only the people as a whole should choose the law, which is then set by the government which acts merely as the arbiter of the people, and arranges for the judges, the police etc. That means a different kind of democracy - not a representative one, but either a true democracy where everybody is heard, or for practicality a sampling democracy where a sufficiently large poll is made, or a sufficient number of people are chose by random to stand as an approximation of the public voice. That puts the politics out of the government, and into the hands of the people who then have to convince each other over policy rather than pay a bunch of career politicians to do their way, because nobody would know who exactly will get to make the call.
