"The blind leading the blind" is an idiom and a metaphor in the form of a parallel phrase which can be traced back to the Upanishads, written between 800 BCE and 200 BCE:
"Abiding in the midst of ignorance, thinking themselves wise and learned, fools go aimlessly hither and thither, like blind led by the blind." — Katha Upanishad
A similar metaphor exists in the Buddhist Pali Canon, composed in North India, and preserved orally until it was committed to writing during the Fourth Buddhist Council in Sri Lanka in 29 BCE:
"Suppose there were a row of blind men, each holding on to the one in front of him: the first one doesn't see, the middle one doesn't see, the last one doesn't see. In the same way, the statement of the Brahmans turns out to be a row of blind men, as it were: the first one doesn't see, the middle one doesn't see, the last one doesn't see." — Canki Sutta (MN 95)
The expression also appears in Horace: Caecus caeco dux ("the blind leader of the blind"). Horace was the leading Roman lyric poet during the time of Augustus (27 BCE – 14 CE).
An expression applied to leaders who know as little as their followers and are therefore likely to lead them astray: “When it comes to science and technology, many politicians know as little as the average citizen; they're the blind leading the blind.” Those lacking the skills or knowledge for something are being guided by equally inept individuals. The expression is found in the New Testament as one of Jesus's teachings (Matthew 15:14; Luke 6:39).
13 Every plant that my heavenly Father has not planted will be pulled up by the roots. Leave them; they are blind guides [of the blind]. If a blind man leads a blind man, both will fall into a pit. Let them alone: they be blind leaders of the blind. And if the blind lead the blind, both shall fall into the ditch.
39 And he spake a parable unto them, Can the blind lead the blind? shall they not both fall into the ditch? A student is not above his teacher, but everyone who is fully trained will be like his teacher.
Interpretation of The Parable of the BlindLike several of Bruegel's other compositions including Hunters in the Snow (1565) and Peasant Wedding (1567), both in the Kunsthistoriches Museum, Vienna, this late work of religious art by Pieter Bruegel the Elder relies on a diagonal spatial arrangement, which is presented here in its most extreme form. One of the greatest Renaissance paintings in its simple rendering of a New Testament parable, the work is the usual subtle Bruegel mixture of genre painting, moralistic sermon and landscape painting. Groups of blind beggars were a common sight in 16th century Europe, and were a subject of regular concern to Bruegel. He incorporated a group of them into The Fight between Carnival and Lent (1559, Kunsthistorisches Museum, Vienna), and a blind individual into The Sermon of Saint John the Baptist (1566, Museum of Fine Arts, Budapest). His painting of them in this work is unsentimental, but so accurate that doctors have been able to recognize several of their eye disorders: the man third from the left is suffering from leucoma of the cornea, while the man in front of him has amaurosis. The painting hangs in the Capodimonte Museum, Naples.
The Free Dictionary:
"Someone who is not capable of dealing with a situation is guiding someone else who is not capable of dealing with it."