Classification is the process of grouping organisms together based on similarities between the various organisms. Taxonomy is the science of classification or naming the groups made up- of similar organisms. A taxonomist is a scientist who classifies or names these groups of organisms. Man has always had a need to organize his world and classification is one of the tools/processes used to group similar objects together and to make "sense" of his environment. One of the earliest schemes of classification was developed by the Greek philosopher, Aristotle. He attempted to classify the living organisms that he saw daily. Aristotle said that organisms were divided into two large, main, groups and were either plants or animals. Animals were further subdivided into air dwellers, land dwellers, or water dwellers. Aristotle also divided plants into three catagories: herbs were plants with soft stems; shrubs were plants with several woody stems; and trees were large plants with one large trunk. Aristotle's classification system was accepted by scientists until the eighteenth century. The invention of the microscope and Pasteur's disproving of spontaneous generatioln showed scientists that many organisms exited which could not easily, or logically, fit into Aristotle's classification system.