"The sensory acts are accordingly distinguished by their objects"

In this quote, Samuel Alexander is referring to the way our senses connect with the world around us. He recommends that our sensory experiences are specified by the items we communicate with - simply put, what we see, hear, touch, and so on is determined by what's actually out there on the planet. For example, if we see a red apple, our visual experience is shaped by the homes of that apple: its color, shape, size, etc. This insight assists us comprehend the value of our environments in shaping our perception of the world, and highlights how carefully our senses are connected to the physical truths of our environment.