We see a car. It looks great compared to a bike, and small compared to a bus. 

We see a 60-year-old person. It seems robust and healthy compared to an old man of 90 years, and weak and morbid in front of a young man of 20 years. 

Many other examples could be added, some on issues as important as beauty, kindness or justice. 

Plato wondered about the origin of notions like the previous ones. Why do we say something is big or small? Why do we declare just one act and unjust another? 

Some will say that there is no such problem, for a day comes when everything will be explained by neurons, or by hormones, or by physical and chemical laws that describe the origin of each idea.

But even if such a day came (something that seems impossible), some will remind the experts that their theory would become true only if something outside it allowed to judge it. 

In the Platonic explanation, ideas cannot be reduced to the sum of material elements, or to components that are part of the sensitive and measurable physical world.

For Plato, ideas are based on a "world" (which called the world or place of ideas) that is beyond the sensible, the measurable, of what is composed of material parts. 

Many will deny such a world, or they will think that ideas have no validity. Paradoxically, rejecting (or accepting) the validity of ideas implies recourse to a new idea, that of validity or correction...

Years ago a writer said, with a kind irony, that we are prisoners of Plato, because it is impossible to talk about any subject without resorting to ideas.

Perhaps it would be better to say that we are beings with something different from material, something spiritual, that allows us to make judgments on goods, on truths, on values, and on so many other dimensions that exceed in much what can be measured with powerful instruments Scientists.

Accepting metaempirical ideas implies recognizing that human beings are located at a level of existence close to that of pure spirits (angels), and even to that of a god who exists beyond any limit and contingency. 

Which becomes a source of great hope (something also taught by Plato), that hope that gives full meaning to our human condition, already during this life and in life that, we hope, will continue beyond death.