Question: List three reasons why Rembrandt's works are difficult to definitively identify.
About the article "Rembrandt", section "Expert assessments".

Candidate passages:
The Southern Cloudywing (sometimes spelled Southern Cloudy Wing), "Thorybes bathyllus", is a North American butterfly in the family Hesperiidae.
There are two main reasons why Southern Cloudywings can be difficult to identify: one is individual variation and the other is confusing seasonal forms.
In the south, where it has two broods per year, there are two seasonal forms.
Spring forms are usually lightly marked and resemble Confused Cloudywings ("Thorybes confusis").
Summer forms tend to be more boldly marked by comparison, making identification easier.
However, summer Confused Cloudywings are also strongly patterned which makes identifying them more difficult.
There rapid flight is very erratic, though it is closer to the ground then in some of its close relatives. 
In 1968 the Rembrandt Research Project began under the sponsorship of the Netherlands Organization for the Advancement of Scientific Research; it was initially expected to last a highly optimistic ten years.
Art historians teamed up with experts from other fields to reassess the authenticity of works attributed to Rembrandt, using all methods available, including state-of-the-art technical diagnostics, and to compile a complete new catalogue raisonné of his paintings.
As a result of their findings, many paintings that were previously attributed to Rembrandt have been removed from their list, although others have been added back.
Many of those removed are now thought to be the work of his students.
One example of activity is "The Polish Rider", in New York's Frick Collection.
Rembrandt's authorship had been questioned by at least one scholar, Alfred von Wurzbach, at the beginning of the twentieth century, but for many decades later most scholars, including the foremost authority writing in English, Julius S. Held, agreed that it was indeed by the master.
In the 1980s, however, Dr. Josua Bruyn of the Foundation Rembrandt Research Project, cautiously and tentatively attributed the painting to one of Rembrandt's closest and most talented pupils, Willem Drost, about whom little is known.
But Bruyn's remained a minority opinion, the suggestion of Drost's authorship is now generally rejected, and the Frick itself never changed its own attribution, the label still reading "Rembrandt" and not "attributed to" or "school of".
More recent opinion has shifted even more decisively in favor of the Frick, with Simon Schama in his 1999 book "Rembrandt's Eyes", and a Rembrandt Project scholar, Ernst van de Wetering (Melbourne Symposium, 1997) both arguing for attribution to the master.
Those few scholars who still question Rembrandt's authorship feel that the execution is uneven, and favour different attributions for different parts of the work.
A similar issue was raised by Simon Schama in his book "Rembrandt's Eye" concerning the verification of titles associated with the subject matter depicted in Rembrandt's works.
For example, the exact subject being portrayed in "Aristotle with a Bust of Homer" (recently retitled by curators at the Metropolitan Museum) has been directly challenged by Schama applying the scholarship of Paul Crenshaw.
Schama presents a substantial argument that it was the famous ancient Greek painter Apelles who is depicted in contemplation by Rembrandt and not Aristotle.
Another painting, "Pilate Washing His Hands", is also of questionable attribution.
Critical opinion of this picture has varied since 1905, when Wilhelm von Bode described it as "a somewhat abnormal work" by Rembrandt.
Scholars have since dated the painting to the 1660s and assigned it to an anonymous pupil, possibly Aert de Gelder.
The composition bears superficial resemblance to mature works by Rembrandt but lacks the master's command of illumination and modeling.
The attribution and re-attribution work is ongoing.
In 2005 four oil paintings previously attributed to Rembrandt's students were reclassified as the work of Rembrandt himself: "Study of an Old Man in Profile" and "Study of an Old Man with a Beard" from a US private collection, "Study of a Weeping Woman", owned by the Detroit Institute of Arts, and "Portrait of an Elderly Woman in a White Bonnet", painted in 1640.
Rembrandt's own studio practice is a major factor in the difficulty of attribution, since, like many masters before him, he encouraged his students to copy his paintings, sometimes finishing or retouching them to be sold as originals, and sometimes selling them as authorized copies.
Additionally, his style proved easy enough for his most talented students to emulate.
Further complicating matters is the uneven quality of some of Rembrandt's own work, and his frequent stylistic evolutions and experiments.
As well, there were later imitations of his work, and restorations which so seriously damaged the original works that they are no longer recognizable.
It is highly likely that there will never be universal agreement as to what does and what does not constitute a genuine Rembrandt.
"S. caprae" occurs as a commensal on human skin, but has also been implicated in infections of the bloodstream, urinary tract, bones and joints.
Because "S. caprae" is difficult to identify definitively in the laboratory, it is likely that it occurs more often than the medical literature suggests. 
Win–loss analytics involves identifying and analyzing the reasons why a visitor to a website was or wasn't persuaded to engage in a desired action (conversion).
This information allows web teams to improve the website's navigation and content, identify individuals that are more likely to convert, to improve marketing efforts. 
Juvenile Dilated Cardiomyopathy is a rare, fatal condition caused by an autosomal recessive gene.
It affects young dogs, who succumb to heart failure before reaching adulthood.
As a simple recessive gene, it had been difficult to identify and was particularly heartbreaking as seemingly healthy puppies would suddenly die, often shortly after joining their adopted families.
Since a recessive gene is responsible, that means if at least one parent is homozygous dominant (that is, it does not carry a copy of the cardio version of the gene), its offspring can not contract the disease.
In 2007 a genetic linkage test became available which appears promising.
This is not a test which confirms if a dog has, or does not have the disease; nor will it definitively predict the disease, as even if a dog is a JDC carrier this does not guarantee its offspring will suffer the disease.
It only links certain strains of DNA as carriers of JDC.
This is significant in that these strains can now largely be deselected for in the breeding process, as has been successful with GM1 Storage Disease (see above).
The test is not yet complete for every bloodline, and "why" the identified strains are implicated is still unknown, and so in essence the "cause" of the condition remains a mystery to be solved.