People often see hidden faces in things. Depending on the circumstances, this is referred to as pareidolia, the perception or recognition of a specific pattern or form in something essentially different. It is thus also a kind of optical illusion. When an artist notices that two different things have a similar appearance, and draws or paints a picture making this similarity evident, they make images with double meanings. Many of these images are hidden faces or hidden skulls. These illusionistic pictures present the viewer with a mental choice of two interpretations: head or landscape, head or objects, head or architecture, etc. Both of them are valid, but the viewer sees only one of them, and very often they cannot see both interpretations simultaneously. Chance images There are everyday examples of hidden faces, they are "chance images" including faces in the clouds, figures of the Rorschach Test and the Man in the Moon. Leonardo da Vinci wrote about them in his notebook: "If you look at walls that are stained or made of different kinds of stones you can think you see in them certain picturesque views of mountains, rivers, rocks, trees, plains, broad valleys, and hills of different shapes. You can also find in them battles and rapidly moving figures, strange faces and costumes, as well as an infinite number of things." Francois and Jean Robert collected and published a lot of photos of "chance faces". Hidden faces created by artists The Mannerist master at the 16th-century imperial Habsburg courts of Vienna and Prague, Giuseppe Arcimboldo of Milan was probably the best known artist for creating extraordinary hidden faces. He arranged flowers, vegetables, fruits, shells, scallops and other animals, books and different things on the canvas in such a way that the whole collection of objects formed a portrait. His series of The Four Seasons seems to be the first use of this approach and technique. Arcimboldo's composite heads were celebrated and imitated by his contemporaries but they were relatively forgotten until participants in the twentieth-century art movements rediscovered them, bringing them to the attention of art historians. He is considered as forerunner of Dada and Surrealism. Some other famous Renaissance and Baroque artists created hidden faces: Tobias Stimmer Hans Holbein the Younger Matthäus Merian Maria Sibylla Merian Marcus Gheeraerts the Elder Wenzel Hollar Josse de Momper Salvador Dalí was fascinated by the technique of Arcimboldo and his paranoia-critical method