What is "current source"? Are there true current sources? If not, how do we create artificial current sources? How do we make them perfect?

For some reasons, voltage sources are widespread in this world while it seems there are not true current sources. So, we should build them on the base of voltage sources. It is interesting to consider and compare the various techniques for creating current sources in circuitry...

We have already considered some aspects of this problem in the questions below:
https://www.researchgate.net/post/Are_there_causal_relationships_in_Ohms_law_If_so_which_is_the_cause_and_which_is_the_effect
https://www.researchgate.net/post/Voltage_or_current_controlled_device_is_a_BJT_And_what_does_voltage_controlled_and_current_controlled_mean_Is_this_a_property_of_the_BJT?
https://www.researchgate.net/post/What_is_the_basic_idea_behind_the_BJT_current_mirror_Is_it_a_reversible_device_can_we_swap_its_input_and_output_and_why

I have showed some popular techniques for creating current sources in the stories below; you can use them as an initial point of the discussion:
http://www.circuit-fantasia.com/circuit_stories/inventing_circuits/current_source/current_source_philosophy/current_source_philosophy.htm
http://en.wikibooks.org/wiki/Circuit_Idea/Simplest_Transistor_Current_Source
http://en.wikibooks.org/wiki/Talk:Circuit_Idea/Simplest_Transistor_Current_Source
http://en.wikibooks.org/wiki/Circuit_Idea/Op-amp_Inverting_Voltage-to-Current_Converter
http://en.wikibooks.org/wiki/Talk:Circuit_Idea/Reinventing_the_Famous_Howland_Current_Source
http://en.wikibooks.org/wiki/Talk:Circuit_Idea/Deboo_Integrator
http://en.wikibooks.org/wiki/Circuit_Idea/How_to_Make_a_Perfect_RC-integrator

I would be glad if you add more current source techniques.