If you would have the size of an electron, no objects like cars or clouds would exist for you. If you would live a billion times slower, animals would look like sparks for you. If you would have the density of a black hole singularity, objects you swallow with your humongous gravity might appear as sound waves inside your body.
In synthesis, there are no objects as such in nature. When you interact with an object, it is your speed, location, size, hardness, density, chemical composition that determine what objects exist for you. Since you have approximately the same chemical composition, speed, size, density, etc. as me, we can interact with almost the same objects and even talk about objectivity! (our ability to focus on the same apparent features of the object).
In general, you can think of objects as only part of a continuum of fundamental forces. There are no cars or rainbows outside of our heads. We're not even sure that time and space are real.
Immanuel Kant was a philosopher who suggested that the subject defines the object, which is essentially the same idea as above, but Kant focused mainly the rational aspect.
In philosophy, there are quite different perspectives, let's see some:
On the extreme non-existence of objects, there is solipsism (see George Berkeley), which more or less states that nothing exists, that everything is a divine projection of God into our minds.
Kant is on the middle, he speaks of the thing-as-it-appears and the thing-in-itself.
On the other extreme, there are multiple perspectives, but the most remarkable is scientific realism, that is... science (!), which takes objects maximizing objectivity, and minimizing subjectivity to the point that the subject mostly does not exist in science (except as an OBJECT (of study), which is really ironic). I say that scientific pure objectivity implies nullifying the subject. For scientific realism, even without humans, scientific knowledge should hold (which will possibly not for an ethereal living being). Excepting modern sciences like quantum mechanics, few disciplines really address the influence of the subject in interactions (think on this: ALL disciplines of knowledge are about the interaction of the subject with some type of object; geology is about how WE know rocks, dentistry is about how WE address dental issues, psychology is about how we address other subjects (which for the practitioner is always an object, he is the real subject).
Objects exist only while our subjective capabilities (mental and physical) hold. We shouldn't take for granted that other forms of objects or even living beings in the universe exist only within our scales: they don't necessary have the same size, speed, density, composition as us. Perhaps rocks interact physically but not on the fundamental fields we can perceive. Perhaps mountains are intelligent, perhaps there are animal-like entities in dark matter. Perhaps a 2cm martian started going through your body ten minutes ago, and neither you nor him notice the other.
