Limiting the existence of something to the ability to reach it is not correct. If you could prove that we cannot have visitors from other time that would be a solution. According to the present state of theory one can invert time and no law of physics will be broken. Thus the feasibility of time...
Why should point have no definition? I guess it has a pretty good definition in integral calculus as a device for integration of 0-dimensional infinitesimally small quantities (points in algebraic and geometric sense) into multidimensional objects. 1-D integral will give a path, which is still...
I came across of an equality which I have difficulty to understand. If f_n is a rational algebraic homogeneous function of degree n in the differential operators and if g_n is a regular non-differential homogeneous function of the same degree n, following equality takes place [Hobson: The theory...