This document includes the introduction to and unit A of "Idea-Centered Laboratory Science" (I-CLS). The introduction describes the objectives, the basic assumptions and suggested teaching procedures of I-CLS. The basic theme of unit A is "How a Scientist Studies His World." Laboratory experiences consist of investigations into: (1) idea of observing, (2) idea of asking questions, (3) idea of hypothesis forming and testing, (4) idea of objectivity and tentativeness, and (5) idea of natural law. The 18 laboratory experiences in this unit, as in all I-CLS units, are inquiry related and designed primarily to develop an understanding of how a scientist expects his world to behave. The format for each laboratory experience is as follows: Introduction, Materials and Equipment, Collecting Data, and Follow-up. (BR)