The understanding of photosynthesis developed gradually over nearly two centuries of experimentation, with each scientist building on the discoveries of predecessors. The story begins with the "phlogiston theory" of the 17th-18th century, which proposed that burning substances release "phlogiston" into the air, and that air becomes "phlogisticated" (saturated with phlogiston) during combustion or respiration, eventually unable to support further burning or life. Priestley's work, while interpreted within the phlogiston framework of his time (he described plants as "dephlogisticating" the air), was actually demonstrating oxygen production and CO2 consumption — discoveries whose proper chemical interpretation came later with Lavoisier's oxygen theory of combustion. The progressive refinement from Priestley's qualitative observation that plants "restore air" to the modern molecular understanding of photosynthesis as light-driven oxidation of water coupled to reduction of CO2, mediated by two photosystems and producing ATP and NADPH, represents one of the great intellectual achievements in the history of biological and chemical science.

Joseph Priestley performed several elegant experiments demonstrating that plants produce a gas that supports combustion and animal respiration. In his most famous experiment, he placed a mint plant (Mentha sp.) in a sealed glass vessel with a burning candle. The candle extinguished as the oxygen in the sealed jar was consumed. After several days with the plant in the jar, he found that a new candle placed in the jar could be reignited and would burn — the plant had restored the air. In a parallel experiment, he placed a mouse in a sealed jar: the mouse eventually became insensible (as oxygen depleted). When he placed a plant in the same jar along with another mouse, the mouse survived for much longer. Priestley interpreted these results within the phlogiston theory available to him, concluding that plants "dephlogisticate" air (remove phlogiston). The correct interpretation — that plants produce oxygen — came from Lavoisier's work identifying oxygen as a distinct gas and disproving phlogiston theory. Priestley also independently discovered oxygen gas (which he called "dephlogisticated air") in 1774 by heating mercuric oxide, though the significance of oxygen in photosynthesis was not immediately apparent.

Jan Ingenhousz, a Dutch physician working in England, substantially extended Priestley's findings in 1779 with a crucial additional observation: plants purify air only in the presence of LIGHT. In a systematic series of experiments, he placed aquatic plants (primarily Myriophyllum and Anacharis/Elodea) in water under different light conditions, observing the production of gas bubbles (oxygen) from the plants. In bright sunlight, numerous bubbles appeared; in shade, fewer; in darkness, no bubbles were produced. He also noted that in darkness, plants actually consume oxygen (similar to animals during respiration). These observations established two crucial principles: photosynthesis requires light energy, and only the green parts of plants perform it (Ingenhousz also tested non-green plant parts and found they did not produce oxygen in light). Ingenhousz published his findings in "Experiments upon Vegetables" (1779), a landmark publication that established the role of light and green tissue in what would eventually be understood as photosynthesis.

Jean Senebier (1742-1809), a Swiss botanist, demonstrated around 1782 that plants absorb "fixed air" (CO2, as identified by Black) during the process of purifying regular air. He showed that plants submerged in water containing dissolved CO2 produced more oxygen than those in CO2-depleted water, establishing CO2 as a substrate for photosynthesis. Nicholas-Théodore de Saussure (1767-1845) performed careful quantitative experiments around 1804, measuring the masses of plants, CO2 absorbed, and O2 produced, and found that the weight gain of plants exceeded the mass of CO2 absorbed — concluding that water must also be incorporated into plant substance during photosynthesis. Julius von Sachs (1832-1897) provided direct visual evidence of photosynthetic product accumulation in 1862, showing that starch appears specifically in illuminated green tissue and demonstrating that chloroplasts (which he helped characterise) are the sites of photosynthesis. These successive discoveries built the conceptual framework that photosynthesis involves: light + CO2 + H2O → organic matter (starch/sugars) + O2.