Where Plato's Academy pursued philosophy chiefly through dialogue and argument aimed at abstract Forms, Aristotle's Lyceum pursued it through what would now be called empirical research, collecting, comparing and classifying the natural world before drawing general conclusions from the evidence. Aristotle and his students gathered specimens and reports covering hundreds of animal species, dissected what they could, and compiled comparative accounts of governments across dozens of Greek city states, work that survives today only in the single case of Athens. This method rested on Aristotle's own logic, the system of the syllogism he developed in his logical treatises, which set out formal rules for valid reasoning that remained the standard teaching of logic in Europe until the nineteenth century.
He also divided knowledge itself into a lasting scheme of theoretical sciences aimed at truth for its own sake, practical sciences aimed at right action, and productive sciences aimed at making useful or beautiful things, a classification that shaped how universities organized their faculties for centuries afterward.
Much of what survives under Aristotle's name are not polished books meant for publication but lecture notes, dense and unevenly finished, edited and arranged by later scholars including Andronicus of Rhodes roughly three centuries after Aristotle's death, which is part of why the prose is often difficult and the argument is often compressed.