Books bear him up a while, and make him try
To swim with bladders of philosophy.

Rochester’s couplet mocks the idea that book-learning and “philosophy” can keep a person afloat amid the pressures of life. The image is deliberately deflating: bladders (inflated animal skins used as makeshift floats) may buoy someone briefly, but they are a flimsy substitute for real ability to swim. The implication is that abstract systems and secondhand wisdom can provide temporary confidence or consolation, yet fail when tested by experience. In typical Restoration-satiric fashion, the lines also jab at intellectual pretension—those who rely on fashionable philosophy may look equipped, but their support is artificial and precarious.