  In sum, in considering Descartes' answer to how we know,  we can distinguish classes of knowledge that differ as regards the degree of certainty one may expect to achieve. Metaphysical  first principles as known by the intellect acting alone should attain absolute certainty. Practical knowledge concerning immediate benefits and harms is known by the senses. Such knowledge is usually good enough. Objects of natural science are known by a combination of pure intellect and sensory observation: the pure intellect tells us what properties bodies can have, and we use the senses to determine which particular instances of those properties bodies do have. For submicroscopic particles, we must reason from observed effects to potential cause. In these latter cases, our measurements and our inferences may be subject to error, but we may also hope to arrive at the truth.