Want to understand ke formula without having to accept any assumptions

To prepare for a future physics course, I googled Newton’s Laws and then found about the Work Energy Theorem. I can see how Newton’s Laws makes sense but I don’t get why the kinetic energy formula is correct. The only experiment for the Work Energy Theorem I can find does not meet the requirements of the scientific method unless you make an assumption. Can someone show me why the work energy theorem is valid without making any assumptions.