Logic line of introductory course of quantum physics is presented. The course is based on Feynman's sum-over-paths theory and now is being offered by E.F. Taylor for high-school teachers and other students at the MIT in the USA. Vector algebra and, high-school mathematics are all the mathematics needed (no partial differential equations). The treatment starts with photons, which obey the fundamental principle: Explore all paths. The basic features of light (reflection, refraction, dispersion, interference...) are analysed just using this principle together with addition of quantum arrows. The unique treatment continues with nonrelativistic description of the behaviour of electrons. First, the classical action for a segment of a path is derived. The actions together with the principle of least action provide seamless transition between quantum and classical mechanics. Then, the wave function, the propagator and description of bound states arise naturally from the application of the simple principles. Students use computers, which help them come round complicated mathematics and often provide a deeper sense of the quantum phenomena.